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    "source_title": "Encyclopaedia Britannica (1926)",
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    "title": "GEOLOGY",
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    "verified_text": "the progress of geology suffered | from a four years’ break, which provided, however, a stimulus to economic geology by the compilations on mincral resources required during the peace negotiations. and to monographs on little-known arcas such as the war fields (die nricgsschau- plitze rorq-1918 scologisch dargesicllt, berlin, 1923-5). the accumulation of geological evidence has increased apace owing to the foundation of new universities, the fuller educational employment of research work and the wider recognition of the economic value of geology. its hterature has become over- whelming, while its bibliographies and indexes are less efficient owing to the increasing bulk of material and cost of publication. one of the urgent needs in geological development is co-operation between the egencies which publish synopses of hterature to secure prompter and fuller references and less serious omissions. in the progress of geological philosophy since 1910 three features have been especially remarkable. first, the icssened over-emphasis on the geological conditions of northwestern europe and the castern states of north america, with the consequent abandonment (cf. e.g., g. a. j. cole, rep. brit. assec., 1915) of that ultra-uniformitarianism which had developed in {zz those countries. second, release from the restricted range of geological time that had been declared necessary from physical and mathematical data. instead of a possible maximum of ten million years, as maintained by tait, geologists are at liberty to assume 8 to ro thousand million years, and need not discard hypotheses which demand prolonged time for minor geological phenomena. third, recognition of the plasticity of the earth’s crust, and of the world-wide effect of earth movements on the relations of land and water. the physical evidence has been found to afford a more precise correlation at some dates than that based on fossils. the belief in the plasticity of the crust and the lateral movement of the land masses has led to the study of mountain ranges and crustal movements with renewed zeal, and to a flood of speculative hypetheses which are perhaps a reaction from the mental strain of the war. origin and constitution of the earth the discussions on the origin of the earth have marked the steady wane of the nebular hypothesis of laplace. the hypothe- ses which are replacing it agree in accepting the solar system as derived from one aggregation of matter; but they attribute its start to the tidal disruption of the parent mass by a passing star. this theory has been put forward in various forms. its modern development is based upon the planetesimal hypothesis, which has been restated by t. c. chamberlin in his origin of the furth, 1916 (and in jour. geol.,| chicago, 21-29); it has been adopted by j. h. jeans, problems of cosmogony and stellar dynamics, 1919, and harold jeffreys, the earth, 1924. according to jeans the parent mass extended throughout the range of the solar system, while. according to jeffreys it was restricted to the inner part. the material of the earth on each of the theories was once loose and was welded by heat due to collisions or pressure. that the original material was gaseous is urged by eddington, since so many stars have the size appro- priate to a gaseous body; but that the earth was constituted by the aggregation of solid meteoritic material agrees with the well- established facts of the earth’s high internal heat, zonal structure, and composition. the planctesimal theory has undergone im- portant developments and modifications in recent memoirs by its founder, t. c. chamberlin. he calls attention (/.g., 28, 1920, pp. 144-6) to the possibility of the planets still receiving matter from the sun, in consequence of such solar explosions as those which in may and july 191g hurled material to the height of at least 475,000 m. above the sun’s surface, and probably outward to the orbits of some planets. the planetesimals, according to chamberlin’s estimates of 1920 (j. g., 28, pp. 672, 681; 29, pp. 407-8) were originally very minute, and weighed about one-fifticth of a pound apiece; amongst other phenomena he attributes to them the zodiacal light and saturn’s rings. he compares them (j. g., 29, 1921, pp.407-9) with the chrondrules, the rounded grains found in meteorites, rather than with the meteorites themselves. they therefore approximate to the meteoritic dust, which, according to t. j. j. see, was once spread throughout space. in spite of the small size of the planetesimals chamberlin regards them as having contributed two-thirds of the mass of the carth by their fall from space as rain upon the original core (j. g., 28, p. 681). jeffreys, however, denies the possibility of any great accretion of such material, and dismisses the planetestmal hypothesis as no solution of any of the main problems of geophysics, and as quite unacceptable on cosmogonic grounds. he has brought it for- ward in a modified form, which, according to chamberlin (j. g., 32, 1924, p.713), accepts all the essential doctrines of the plancet- esimal hypothesis. meteoritic theory.—lockyer, the founder of the metcoritic theory, recognised that many of the meteorites which formed the earth had planetary orbits, and that they came from a single source or zone is supported by the work of sir william crookes (phil. trans., 1918, vol. 217a, pp. 427-30). according to crookes the stony meteorites consist of the elements which form the bulk 1 this journal is referred to in subsequent parts of the article as j. g: ghology of the earth’s crust, and he suggests, from the striking similarity in the proportions of their four chief constituents, that they were all derived from the disruption of a planet that once existed be- tween mars and jupiter; the nickel-iron meteorites may have been derived from the core of the same planet, or may have come from a different source. the chemical similarity of the core of the earth -to the iron meteorites is asserted by l. h. barnett (j. g., 32, 1924, pp. 615-35) as according to him the core consists of 90% iron, 7% nickel, cobalt and copper, and 3° of the other elements. the elementary constitution of the earth has been recalculated and its agreement with a meteoritic composition follows from tamman’s (1924) estimate that 98° is composed of 7 clements in the following order of abundance—iron, oxygen, silicon, magnesium, nickel, calcium and aluminium; four others sulphur, sodium, chromium and potassium, constitute another 1.6%; all the other elements together amount to only four parts per 1,000 of the substance of the earth. the stony meteorites, so far as is known, correspond only to the basic igneous rocks. the occurrence of acid meteorites has been claimed from the obsidianites of australia, but their microscopic structure indicates their formation as acrial ful- gurites by the fusion of dust by lightning during dust storms. zonal structure of the earth.—the earth consists of a series of layers, and much light has been thrown on their constitution and thickness. the bulk of the earth consists of a nickel-iron mass, the barysphere, which is enclosed by a rocky crust, the litho- sphere. within the barysphere is a core, 1,600 m. in diameter, which, according to the late c. g. knott, is fluid, as it docs not transmit distortional waves. in spite, therefore, of the oft- repeated conclusion that the whole earth is solid, scismology proves this large central core to be nuid. between the barysphere and the lithosphere, moreover, according to various authoritics, is 2 weak plastic layer, the asthenosphcre of barrell (j. g., 22, 1914), or the fluid layer of basalt glass which, according to daly clmer. jour. sci., 1923, pp. 347-71), lies below a holocrystalline rigid shell 25 m. in thickness. the lithosphere has been subdivided into various zones. sucss adopted two: the basic sima, composed predominantly of silica and magnesia, and the lighter, more acid sial, composed predominantly of silica and alumina. v.- m. goldschmidt (geochemische vwerteilungsgesetz der elemente, vid. seisk. skr., afath.-nat. kl., kristiania, 1923-24) considers that the nickel-iron core is surrounded by a sulphide and oxide zone which is com- parable in metallurgical operations to a matte; above that ore zone is a stony zone which is comparable to a slag, and is sub- divided into a lower layer of dense silicates, and an upper layer of light silicates and silica. e. d. williamson and l. h. adams (jour. washington acad. sct., vol. 13, 1923. pp. 413-28) divide the lithosphere into three zones; the lowest consists of heavy silicates mixed with nickel-iron and is comparable to the meteor- ites of the pallasite type; this layer passes down into the bary- sphere and upward into heavy silicates, which are covered by a shell of light silicates and silica. that the barysphere is composed of nicke]-iron was affirmed by milne from the acceleration of earthquake waves, toward which it behaves as a rigid material; but under the influence of the intense heat and pressure, the material must at that depth be capable of flow into any cavities like a fluid, so that it has been described as elastico-rigid, or ‘ fluidible.’ the work of f. d. adams (j. g., 20, 1912, pp. 97-118) has shown that the esti- mates of van hise as to the depth at which all rock material would be fluidible must be extended; for adams, by an experi- ment suggested by sir charles parsons, found that small cavities remained open for months in a granite cylinder which was later- ally supported at conditions comparable to those in the earth, at a depth of 11 miles. these experiments, however, deal only with a short period and small cores in a rigid case; they do not represent the conditions of large masses acted upon by immense pressure at a high temperature for jong periods. the high rigidity of the surface of the earth against tidal deformation was supported (j. g., 27, 1919, pp. 585-601) by the observations of michaelson and gale on a horizontal waterpipe 502 {t. long. geology the limited depth of the lithosphere and the constitution of the barysphere of nickel-iron have been supported by the evi- dence of radioactivity, for the earth’s activity is of a strength which can be explained by the restriction of radioactive ma- terials to a depth of about 40 miles. the material of the nickel- iron meteorites is the only non-radioactive substance that is likely to form a considerable part of the earth. the inner earth is assigned a more moderate temperature than formerly; that of the primitive sun, according to jeffreys, cannot have been less than 3,000° c., but within the earth the maximum may be much lower, for the temperature may be nearly uniform in such a good conductor of heat as the nickel-iron core. the depth of eartiiquake origins knowledge of the physical nature of the earth’s interior has made most progress from investigation of earthquakes, most of which have been attributed to a depth of 5 or 6 m.; dr. oldham (quar. jour. geol. soc., 78, 1922, p. 57) concludes that the great majority arise less than 10 m. from the surface. lately, however, physicists have assigned their origin to much greater depths. thelate g. w. walker (rep. brit. assn., 1917, pp. 13-4 and phil. trans. a.222, 1922, pp. 45-56), from the angle, of emergence of earthquakes recorded by the galitzin seismographs at pulkova, inferred that some originate at least 800 m. deep. this deep-seated source has been supported by prof. turner (rep. brit. assn., 1923, pp. 283-6) who assigns the majority toa zone bounded below at a depth of 145 m., and some to a depth of 300 m., such as the formosa earthquake of april 14 1906, at 280 miles. earthquakes.—the word earthquake originally meant a shaking or quaking of the earth which could be felt, might cause damage and was occasionally accompanied by changes in the aspect of the sur- face of the earth. seismology was the department of science which dealt with these disturbances, and was naturally regarded as a minor section of geology; with improved methods of observation and the invention of special instruments the scope of this subject has been enlarged, and it has come to include much that does not belong to earthquakes in the original sense of the word, or to geology, yet cannot be entirely separated. recent developments in the study of the carthquake, which might be dealt with here, are, therefore, transferred to the article earthquake. the shallow origin of some earthquakes was concluded by milne from the twisting and breaking of telegraph cables by slips of material down submarine slopes. it has been found that such breaks are not recorded on seismographs, so that they cause no widespread vibration in the underlying rocks. the main claim for the superficial origin of a world-shaking shock, other than those due to volcanic explosions, is that by the late prince galitzin, who referred an earthquake, on feb. 18 1911, to a jand- slip in the pamir. a mass of earth weighing 7,500,000,c00 tons fell into the valley, buried a village and all its inhabitants, and formed a bank over 3 m. long and 2 m. wide, and a lake 15 m. long and goo ft. deep. dr. oldham in a rediscussion of the evidence (quar. jour. geol. soc., 79, 1923, pp. 237-45) has shown that the landslip was a result of the earthquake and not its cause, for it was not above the centre of the disturbance, which he assigns to a depth of 30 miles. volcanic action volcanic studies have been greatly advanced by work at the observatory on kilauea in the hawaiian islands under the direction of t. a. jaggar. his weekly letter enables geologists in all parts of the world to follow the changes in that instructive volcano. its cauldron was enlarged by huge subsidences in 1919, 1922 and 1924, the last being associated with powerful ex- plosions. : the destructive eruption of vesuvius of 1906 has been de- scribed by perrett (publ. carnegie institute, no. 339, 1924), who shows that the weight of the lava in the throat of the vol- cano broke through the wall, and the relief of pressure on the molten rock enabled its superheated water to explode; the ex- plosion threw up a column of vapour 7 m. high and produced avalanches of hot volcanic sand like that from mt. pelee, which overwhelmed st. pierre, the capital of martinique, in 1902. 173 the pelean eruption has been further brought into line with ordinary volcanic phenomena by the eruption of katmai, at the base of the alaska peninsula, in june 1912—the most notable volcanic event of recent years. katmai is an old volcano of basic andesite resting upon jurassic sandstones. the erniption was of the paroxysmal explosive type. it scattered 5 cu. m. of pumice and scoria over the district, depositing a layer a foot thick even 100 m. away. ‘the eruption left a great hollow with nu- merous steam vents on its tloor; the summit of the mountain was replaced by a pit 2} m. wide, and from 2,000 to 3,700 ft. deep. this crater was at first attributed to the explosion, but c. fenner j.g., 28, pp. 569-606, 1920) shows that it was due to subsidence, aided by the solution of the older andesites by the ror2 lava. the eruption is explained as due to the intrusion of a sheet of rhyolite under the volcano. the intrusion shattered the over- lying rocks and the rhyolite rose through the fractures. the katmai obsidian, when heated, froths up to pumice, and when it reached the surface it was blown into fragments by its super- heated steam. this eruption has explained two phenomena of the west indian eruptions of 1902 that had appeared unique. part of the katmai area was covered with beds of sand, composed of pumice that had been pulverized by the explosive escape of the steam. this sand is of the same nature as the incandescent dust which fell in an avalanche upon st. pierre. a new secondary peak named nova rupta was formed at katmai by the forcing of a dome of rhyolite into the overlying rocks; it is similar to the lava spine which was pushed up through the vent of mt. pelee. volcanic cauldrons.—the first report that the great cauldron at katmai was due to an explosion, threw doubt on the origin of some volcanic basins that had been attributed to subsidence. j. s. diller (7. g., 31, pp. 226-7, 1923) has restated the evi- dence proving that “‘ crater lake ” in oregon lies in a basin due to subsidence. it is therefore a cauldron. this origin has been attributed to some huge volcanic basins in east africa, such as menengai in kenya colony, and ngorongoro which has been further described by barns in his across the great crater land (1923). mount aso in japan remains by a little the largest known cauldron, being 14 m. long by 10 m. wide (anderson, j. g., 16, p. 499, 1908). suill larger subsidences form sunk- lands in some of which, such as the riessenkessel in southern ba- varia, the sunken rock, there a shattered granite, is surrounded by a ring of lava. the ring-shaped intrusion of syenite, 16 m. in diameter, at pilansberg in the transvaal, described by w. a. ifumphrey (1914), represents the deeper part of such a formation. the importance of such ring-dikes in volcanoes has been shown in the island of mullin western scotland. its central mountains are the deeply dissected foundation of a volcano of the first order; and the tendency of lavas to rise along circular fractures is there well displayed and has been described in geological survey afem- oiy (by e. b. bailey and others), which confirms the view of judd that the ancient volcanoes of the western isles were separate volcanic vents. plateau eruptions —that some of the mull lavas were ejected by fissure eruptions is still advocated, but this origin is not shown for any lava left on the island. knowledge of the vast lava fields that have been attributed to fissure eruptions has been extended for those in south africa and south america. those in south africa have been described by du toit, “ karroo dolerites of south africa” (trans. geol. soc. south africa, 22, pp. 1-42), who shows that the stormberg lavas (of which the age is rhaetic or liassic) cover an area around basutoland 350 m. long by 150 m. wide. a surrounding area of some 320,000 sq. m. is seamed by dolerite dikes, so that the volcanic area was doubtless more extensive than it 1s now. c. l. baker, “the lava field of the barana basin, south | america” (j.g., 31, 1923 pp. 66-79), points out that the basalt flows there cover an atea of 300,000 sq. m. with an average thickness of 1,000 ft.; and the surrounding country, to the extent of 75,000 sq. m., is penetrated by innumerable sills and dikes of dolerite. these south american eruptions are of the same or of approximately the same age as those of south africa—namely, at the very beginning of the jurassic. 174 afeleoritic impact crater —the view that coon buttcor metcor crater in arizona was duce to the impact of a great meteorite has failed of further support by the explorations of d. m. barringer. it was suggested that the crater was made by the nucleus of a comet instead of by a single meteorite. recognition that this body struck the carth obliquely led to a fresh search for the meteorite under the southern rim of the crater and not, where previous borings were made, under the centre. recent boring found only country rock. isostasy the most significant discussion of the period has been upon the extent of isostasy, e.¢., bull. geol. soc. :lmer., 33, 1922, pp. 275-410 (see isostasy). the evidence for the greater density of the rocks below the oceans has been strengthened by pendulum observations in a submarine. this high density, according to i’. p. shephard (j. g., 31, 1923) may be due to compres- sion. the view generally adopted is, however, that the rocks below the oceans consist of denser material—the sima. since the occan floor must consist in places of sunken continental rocks, the existence of sima so near the sea bed appears uncertain. isostasy explains how large areas of the earth’s crust may be uniformly uplifted, though the possibility of such a movement was denied by suess. burrard (prof. pap. survey of india, no. 12, 1912, p. 6) has, however, supplied an instance in which 5,000 sq. m. of country between saharanpore and mussurie in northern india were upraised 5 inches during an earthquake. this fact was proved by re-levelling a line that had been surveyed shortly before the earthquake. areas of tension areas that are being slowly upraised are torn by tension clefts and broken by normal faults. faults so weaken the rocks they traverse that the side left upstanding is usually reduced toa gentle slope; nevertheless some recent faults in areas of hard rock are marked by fauilt-scarps which bound sunklinds and rift valleys. of the latter the greatest extends from palestine across fiast africa, and as shown by teale and wilson (1915) south of the zambezi. its geology has been described by gregory (geology and rift valleys of hast africa, 1921) and krenkel (bruchzonen ost-afrikas, 1922). that this valley had a longer history than was originally recognised by suess is now admitted. i. lehmann (zeit. vulkan., erg. 4, 1924) has described the volcanic area in the rift valley north of lake nyasa and shown that its rocks are alkalic in composition. ‘the tectonic origin of the gulf of suez, a branch of the great rift valley, has been proved by the petroleum bulletins of the geological survey of egvpt, and prof. brock has contirmed that view for the dead sea (quar. jour. geol. soc., 75. 1919). abendanon, as a corollary to his view of the racial contraction of the earth, considers that some parts are forced upward by the subsidence of others, and that tension in the rising parts gives rise to rift valleys. w. il. bucher, (j. g., 32, 1924) has illus- trated the importance of tensional structures by model shells in which the plan of the crumpled bands of the earth are produced by tension and not by compression. the deformation of the earth leads to wide areas of subsidence and uplift. thus the subsidence of an ocean floor, by forcing the material below it to flow outward, may uplift its borders. where the uplifted area consists of hard rocks, the surface will be torn by a network of tension clefts forming fords and fiord-valleys (j. w. gregory, the origin and nature of fiords, 1913). the up- heaval of these areas accompanied great subsidences that fol- lowed the crumpling of the earth’s surface during the mountain *formation which culminated in the miocene. where the surface consists of soft rocks the rupture may control the topography, as in the rectangular valley system of north-castern france, and s. w. wooldridge (proc. geol. assoc., 34, 1923, pp. 175-92) sug- gests that many topographic features of the london basin were | determined by intersecting fractures in the underlying platform of old rocks. ‘the tectonic origin of the greenland fiords has been supported by l. koch (j. g., 31, 1923, pp. 42-65) who geology points out that some of the fiord valleys are pre-glacial, while others are of very late origin and have been but slightly in- fluenced by denudation. these fiords he calls fracture-fiords, as they were formed by the cleavage of gneiss along planes of fracture. areas of compression and fotp mountain ciains alpine structure.— intense compression of the crust produces fold mountains, the study of which has been especially active in the alps. it has long been known that some of the northern alps consist of blocks of old rock resting on younger beds. ‘these ‘“ klippe,” or “ mountains without roots ’’ have been pushed sideways on to their present foundations, a process demonstrated in the north-western highlands of scotland. there, in some cases, the rocks were broken into short slabs which were forced one over another, giving rise to the “ imbricate ” structure, such as would be produced if a row of slates on a roof were pressed into a shorter space. in switzerland the process has taken place on a great scale, and successive slabs of rocks have been thrust one over another as decken or overthrust-sheets. in some parts of the alps this structure is indisputable, but in others the evidence depends on an uncertain identification of the rocks. thus monte rosa and the matterhorn have been inter- preted as pinnacles of old rock thrust northward over the trias. the triassic age of the underlying rocks is uncertain, and is a tradition dating from the time when many of the alpine schists were regarded as jurassic. bonney’s work in the lepontine alps showed that the so-called jurassic schists, like the carboniferous ‘ fossiliferous gneiss,’’ were beds composed of schist debris, and were infolled amongst the ancient rocks. the pre-palaeozoic age of the crystalline schists of the alps is now generally adopted, and the triassic age of the base of monte rosa and the matter- horn is uncertain. the supposed overthrust parts of those moun- tains have no similar rocks to the south, so that they must have been transported from afar; and some advocates of the extreme decken theory regard parts of the overthrust sheets or “‘ nappes ” as african in origin. this view is dismissed as fantastic “ ultra- nappismus ” by some alpine geologists, and has led heritsch to describe the decken theory as a fantasy. the distance to which sheets of rock can be thrust is limited by their crushing strength. according to a. c. lawson the mechanical limit of overthrusting must be between 20 and 30 m., and oldham has adopted a limit of about 5 miles. an overthrust of iso m. has been claimed in scandinavia but appears impossible and is unnecessary on a more probable correlation of the rocks. the decken theory tn its extreme form may prove untenable. amongst the voluminous literature on the theory reference may be made to albert heim’s work, geologie der schweiz (1918-22); to the case for it by r. staub, der bau der alpen (1924); for a critical view to i. heritsch, die grundlagen der alpinen tek- tovik (1923). the investigation of alpine movements has shown that they began earlier than was realised. the movements culminated in the oligocene and miocene; but the preliminary buckling be- gan early in the mesozoic. increasing importance is attached to the younger granites of the alps, to which fresh attention has been directed by steinmann. movements of the decken type in the southern and eastern highlands of scotland have been ad- vocated by fe. b. bailey (1910-25); but his interpretation has not been generally accepted. the decken theory of the alps agrees in one respect with suess’s interpretation, by attributing the main folding to pressure from the south. that direction of movement has been rejected by l. kober, buu wad entstehung der alpen (berlin, 1923), who denies the asymmetry essential to suess’s view that the alpine belt has been pushed northward and buckled against the resisting masses of central france, the black forest, bohemia and the russian platform. kober regards the alps as symmetrical; he explains alpine mountains as due to a belt of the crust being crushed between the continental coigns, as they are forced to- gether by the earth’s contraction. he regards the mediterranean basin as one mountain block or “ orogen,”? which has been geology crumpled between northern europe and north africa, the move- ments being outward from the compressed belt and therefore northward in the alps and southward in the atlas. in the atlas the pre-alpine folds are southward; but those of the alpine period, according to gentil, are northward. that direction is in- consistent with kober’s requirements. longwall has pointed out that the structure of the sierra nevada of california is also inconsistent with kober’s explanation of the rocky mountains; and hobbs has rejected the theory for the asiatic arcs, which he attributes to deep-seated pressure from the rear or concave side. the contraction of the earth —the view that mountain folding is due to the compression of a contracting earth has been further discussed; arguments against it have been stated by sandberg (1924) and keith (bull. geol. soc. amer., 34, 1923); but it has been more widely adopted owing to the recognition of the fallacy of some objections. it was claimed that the utmost possible shrinkage of the earth was inadequate for the observed com- pression; that view was based on the limited age of the earth, but with the multiplication of its age 10 or 20 times that argument has become invalid. jeffreys has pointed out that shrinkage by cooling would be sufficient, and there are other even more potent factors of contraction. t. c. chamberlin has supported the shrinkage of the earth’s crust from the greater density of the earth as compared with the moon. if both originally consisted of similar material, the higher specific gravity of the earth is due to condensation, and to the shrinkage of its circumference by 4,555 miles. the number of mountain chains in which overthrusting is rec- ognised has been increased, as by the work of deprat in tonkin and southwestern china (fem. geol. serv. indo-china); his con- clusions in this respect have been confirmed by jakob. over- thrusting has been recognised in the areas between indo-china and the eastern end of the himalayas in northwestern yunnan (phil. trans., vol. 213b, pp. 171-298, 1925). other mountains, however, though folded, show but slight overthrusting. thus the andes, though gently folded in the upper cretaceous and eocene, have subsequently undergone block faulting without thrust planes, as shown by steinmann (geol. rundsch., 1922) bosworth (geol. north-west peru, 1922), douglas (quar. jour. geol. soc., 1914, 1920, 1921) and h. backlund (1923). different parts of one mountain system may have been affected by both types of movement; thus the northern rocky mountains are shown by mansfield (1923) to have undergone intense lateral compression, while lee (1923) and r. t. chamberlin (1910) showed that the movements in thesouthern rockies were vertical. narrow u-shaped fotd-lincs.—suess explained some important topographic features by the fold-lines having been diverted in a hairpin-bend, in assam, the caribbean islands, the south georgia arc in the south atlantic, and the moluccas to the west of new guinea. this view has been adopted by hobbs (1925s) and extended for the moluccan arcuate fold by g. a. i’. molen- graall and ii, a. brouwer (1921); but the evidence for these narrow u-shaped bends is inadequate, as the main himalayan line of assam continues eastward into south china; the rocks of south georgia are fundamentally different from those of the andes; in the west indies the contorted foundation of barbados strikes west to east almost at right angles to the supposed arcuate fold; and in the moluccan islands the essential strike is across the direction of the chain, and the depression to the west may be due to a foundered block. tencous rocks and earth movements.—that distinctive series of igneous rocks are associated with different types of earth movement has been reaflirmed by ilarker (-lddress, geol. soc., rot7), but has not been generally confirmed (see for example loewinsson-lessing, bull. geol. soc. france (4), 23) from the russian evidence, and for the australasian area (cf., scientia, ii, 1912, pp- 56-63). the nature of the intrusions of the great plutonic masses has been discussed by daly in his igneous rocks and their origin (1914), with many luminous suggestions. hans cloos of breslau, from work in southwest africa and silesia (der afechanismus licfenlkanischer vorgdnge, 1-21, and abh. preuss. geol. landes- l70 anstalt, n.f. 89, 1923), rejects daly’s theory that the granite masses eat their way into the crust by assimilating the material they replace. cloos, agreeing thereby with suess, concludes that intrusive granite either forces the rocks apart or occupies spaces made during the subsidence of blocks of the crust. some assimi- lation naturally occurs on the margin, but cloos regards it as only a minor incident. according to him the direction of pressure during the intrusion and consclidation of the granite may be inferred from the rifts, joints and dikes in the granite, which are parallel to the general grain of the country. the foliation in the granite he regards as less important, as it expresses only the local pressure. stratigraphical geology in stratigraphical geology the national geological surveys and private workers in all countries have been collecting an immense accumulation of new data. the most notable addition to the geological surveys is that of china, which, with the new geolog- ical society of china, have together made a great contribution to knowledge of that country. among the principles of stratigraphy the problem whether geological evolution has been even and continuous, or periodic by alternate rapid developments and long intervals of repose, has been further discussed. shepard (1923) insists that the rate of progress has been on the whole regular; but advance by pertodic spurts has received wide support, as from t. c. chamberlin (1921, etc.), r. t. chamberlin (1914, 1921), g. stille, die schrumpfung der erde (1923) and prof. j. joly (phil. afag., 1923, pp. 1167-88) and surface history of the earth (1925) from his theory of the periodic re-melting of the foundations of the crust by radioactive heat; and also from the distribution of land and water by tetrahedral deformation of the crust and its spheroidal recovery. the classification of the pre-cambrian rocks has made much progress, especially in canada, though there is no general agree- ment in the nomenclature. the tendency is to subdivide the pre- palaeozoic rocks into a lower division of gneisses and coarse schists, an intermediate division of strongly metamorphosed sediments, and an upper division of sandstones, which are but little altered and may contain traces of organic remains. some of the pre-cambrian limestones, which are especially abundant in the middle division, are often attributed to an organic origin; but the extensive chemical precipitation of carbonate of lime has become more apparent, and some oglitic structure is inorganic as shown by its artificial formation (e.g., johnston and william- son, j. g., 24, 1916) and the crystalline structure of some re- cently formed odlite; van tuyl (idrd., pp. 792-7) on the other hand shows how odlites with an organic structure may lose all traces of it. in stratigraphical geology the process has been especially im- portant in regard to the pre-palaeozcic and the palacozoic. much work has been done by many british geologists on the lower palaeozoic rocks of wales and the welsh border country, and on the zonal study of the carboniferous limestone, in papers published mainly by the geological society of london. important proposals for the reclassification of the lower palaeozoic have been made by e. o. ulrich of the u.s. geological survey, in a paper which, owing to the extreme changes pro- posed, has been issued in the bulletin of the geclogical society of america (23,1912, pp. 261-680) withanote by the director of the survey disclaiming responsibility for its views. ulrich classifies the lower palaeozoic into five systems instead of three, adding the ozarkian and the canadian systems between the cambrian and ordovician. neither system is likely to be welcomed by british geologists, for there is little evidence for them in the british isles. vhe ozarkian system is either absent or is represented only by the upper lingula flags: the canadian system includes the british tremadoc and arenig series. the proposed systems have not been widely accepted in america. the better american representation of this part of the geological record has enabled dr. grabau to settle an old problem in the northwest of scot- jand; he confirms salter’s view that the durness limestone is ordovician, and not lower cambrian, to which it was transferred 176 from its superposition on the lower cambrian quartzites with olenellus. much light has been thrown on some stratigraphical problems by the separation and identification of the heavy constituents of rocks—a method especially advanced by the work of boswell. the kainozoic classification of many areas has been put on a firmer basis by detailed palaeontological work, which has per- mitted the correlation of different sequences of beds in inde- pendent basins, as in california (for example, by b. l. clark, 1921), and in australia and new zealand, largely by f; c. chap- man. man has been traced back into the pliocene by the rostro- carinate implements found in the red crag by moir (see archae- olocy), which, however, like the early palaeolithic implements from the forest bed of cromer, are not universally accepted. a great stimulus to the study of the geological history of man followed the description of the piltdown skull (eoanthropus) by sir a. s. woodward (quyar. jour. geol. soc., 69, 1913), and im- portant skulls have been found in queensland, south africa and palestine (see man, evolution of). claims have again been made for the occurrence of early man in america, but without carrying conviction. the reports published by various antarctic expeditions have thrown much light on glacial processes, strengthening doubt as to the extent to which ice erodes hard rocks, attaching increased importance to the shattering action of frost, and supporting the subaqucous origin of boulder clay from its absence on the antarctic lands and its formation on the floor of the ross sea. (see palaeontology.) palaeogeography the steady advance of stratigraphical geology enables palaeo- geography to be placed on a firmer footing and its data have been summarised in a series of works by theodor arldt, handbuch der palacogeographte (1919). eduard suess’s great work, das antlitzs der erde was completed in toog9, but the comprehension of his views has been much aided by the french translation edited by prof. e. de margerie (completed 1918), with its in- valuable collection of maps and supplementary material. suess’s teaching has led to increased recognition of the funda- mentally different arrangement of ocean and continent in former times, combined with the stability of some areas and the weak- ness of others. e. haug in his 7rai#e (completed to11) has laid stress on the importance of the geosynclinals as mobile bands. c. schuchert (1923) in several luminous contributions to the palaeogeography of north america has also shown the con- tinuous influence of the geosynclinals and has organised symposia which have collected on various problems expert opinion which might otherwise not have been expressed. ruedemann (1923) believes that in spite of the many changes in the earth’s geography three archi-continents have persisted throughout geological time. this view has, however, been criticised by w. j. miller, who considers that it is impossible to distinguish between the pre- and post-cambrian foliling. l. kober has extended his theory of the alpine structure to the world in general; he explains the distribution of land and water as due to the massive resistant coigns having been surrounded by weak belts, which sink in geosynclinals, are then crumpled into fold mountains, and on refoundering may carry down with them adjacent parts of the continental blocks. climahic changes.—in connection with stratigraphy a good deal of attention has been devoted to former variations in climate (see climate), which many authorities refer to changes in the heat emitted from the sun, leading at times to universal refrigera- tion. this view has been strongly advocated by huntington and visher (climatic changes, 1922) and some change of this nature is regarded as probable by jeffreys. prof. coleman (amer. jour. sct., 1924, pp. 298-404) has called attention to the world-wide nature of the refrigeration during the pleistocene glaciation. the leading alternative explanation rests on geographical changes in the arrangement of ocean and continent or in the relief of the land. prof. william ramsay of helsingfors (geol, afag., 1924) points out that a period of high relief produces a cold climate and that the wearing down of the land into extensive peneplains geology produces a milder climate. c. e. p. brooks, evolution of climate (1922), gives important help to this line of explanation by cal- culating the quantitative effects of changes in the position of land and water. of the former glacial periods, the known range of that in the upper carboniferous and permian has been greatly extended, especially in america, in brazil, paraguay and the argentine, and in the eastern united states, where quayle has shown that the till at squantum near boston is a boulder clay of this period. there has been a tendency to attribute all coarse boulder beds to glacial action, though some have now received other explanations. ‘the glaciation in. south australia and central china is generally regarded as belonging to the upper part of the pre-palaeozoic instead of cambrian. darwin’s theory of coral reefs has gained general acceptance, and the main interest in connection with them has been from daly’s view that their up-growth was due to the rise of sea-level owing to the release of water by the melting of the glacial ice sheets. this argument is largely based on many of the coral reefs rising from banks at the depth of 100 fathoms; but those banks and platforms are probably cut down to that level, as it is the lower limit of wave action. the numerous variations in level of raised coral reefs and beaches do not support a uniform world-wide rise of the oceans. the irregularity in the rise and fall of coral reefs has been explained by molengraaff as due to the isostatic subsidence of volcanic areas by the weight of the volcanic materials being favourable to the growth of coral reefs. economic geology perhaps the most marked change since 1910 has been the im- proved status of economic geology, due to recognition of its in- creasing practical service, and to the value of its contributions to academic science. the further study of coals has strengthened the view that anthracite is formed from the same vegetation as bituminous coal, the difference being due to subsequent changes. the enormous increase of output of oil, which has been magni- fied more than threefold in the past 14 years, has roused anxiety as to the duration of the supplies. oil geology attaches less value than formerly to anticlinal structure as indispensable to a prof- itable oil field. oil prospecting has revealed the form and structure of the salt domes, and the oil fields in persia have be- come the most important under british control. the extensive boring stimulated by the high prices of 1916-9 led to energetic well drilling from 1917-22, and to a sensational increase in oil production in california. it is doubtful whether the oil output can be extended to meet the growing demands or even be maintained, although improved methods of extraction of oil from beds will lengthen the life of many fields. the indi- cations of a decline in some of the leading sources of supply have led to increased attention being paid to oil shale, in the expecta- tion that before the end of the century it will be the main source of mineral oil. discussion on ores of magmatic origin indicates that they are relatively unimportant, unless the term be so defined that practically all primary ore deposits are regarded as magmatic. ‘there has also been increasing recognition of the importance of magmatic water—as in the series of papers by 13 american authors (j. g., 32, 1921, pp. 177-225, 292-310, 373-90, 449-71), who agree that part of the water from the hot springs of the united states is of deep-seated origin. the deep flowing wells of east-central australia have con- tinued to decrease in volume, supporting the view that their ilow is due partly to the plutonic water which rises from below and mixes with the water stored in the stratified beds. no single chemical distinction has been found by which the proportion of the plutonic to the meteoric waters in a deep-seated supply can be determined; but, with the prolonged age now accepted for the world, a small annual supply of plutonic water would, in the course of geological time, make an important addition to the volume of the oceans. bibliography.—see in addition to the works cited in the text, j. w. gregory, geology of to-day (1919); j. geikie, structural and field geology (1920); p. lake and r. h. rastall, a text book af geology, 3rd. ed. (1920); sir a. geikie, class book of geology, 6th ed. (1921). (j. w. g.) geology: new theories _ geology: new theories (see 11.638).—in considering the origin of continents and oceans it is important to have a clear idea of the magnitudes concerned. if we imagine a globe a foot in diameter to represent the earth, with all its features in their true proportions, by far the greater part of the ocean will be less than sis in. deep and only its extreme depths will reach iz inch. almost the whole of the land will rise less than iyo in. above the sea and even mt. everest will have an altitude of less than sho inch. leaving out of consideration exceptional heights and depths the difference in level between the surface of the land and the floor of the deep oceans will be less than si, inch. these are the features for which we are trying to account. of the 12-in. globe all that hes more than an inch beneath the surface is under conditions of temperature and pressure which cannot be approached in our laboratories, and we have no experimental knowledge of the behaviour of materials under such conditions. moreover, time is an important factor, and our experiments give but little indication of what may happen under stresses acting for thousands or millions of years. it is not surprising that there is no general agreement as to the causes that have produced the present distribution of land and sea. earlier ideas, —throughout the greater part of the last cen- tury geologists in general believed that our present continents and oceans were only temporary features of the globe. it was easy to prove that much of the land had once been beneath the sea, it was not possible to show that any part had always been land. it was reasonable to suppose that much of the ocean had once been land, it was not unreasonable to imagine that no part had always been sea. in the last quarter of the roth century, however, the idea of the permanence of the ocean basins began to gain ground. it cannot be said that there is any universal agreement upon the question even yet. many geologists are now inclined to ascribe a high antiquity to the pacific ocean but to look upon the atlantic and indian oceans as compara- tively modern. ‘haug and others still suspect the former exist- ence of a pacific continent. if continents and oceans are not permanent the present distribution of land and sea is only an episode in the history of the globe and has no more significance than the distributions in the past. it was not geologists therefore who first endeavoured to formulate general theories to account for the present shapes of continents and oceans. those who made such attempts ignored the changes in the past and their speculations were received with little favour by geologists. the tetrahedral theory.—by far the most suggestive of these speculations was the tetrahedral theory of lowthian green. it was to some extent foreshadowed by him in the edinburgh new philosophical journal in 1857 and was fully elaborated in his vestiges of the molten globe in 1875. yollowing the widely ac- cepted notions of the time, he assumed that the earth is cooling, the interior contracting more rapidly than the exterior, and under the force of gravity the outer crust collapses. fairbairn’s experiments on the crushing of wrought-iron tubes led him to be- lieve that the collapsing sphere will tend to approach a tetrahe- dral form. the corners of the tetrahedron will rise above the water, forming triangular masses of land; the faces of the tetra- hedron will remain covered and will form the oceans. he places one of the corners at the south pole and the other three in the northern hemisphere. the corner at the south pole is the antarctic continent and the opposite face of the tetrahedron is covered by the arctic ocean. the triangular masses of land formed by the other three corners are represented respectively by north and south america, europe and africa, asia and australia, all of them wide toward the north and tapering toward the south. the two last are united in the north, but the caspian depression is below sea-level and the plain of the obi but little above it. between these three land-masses lie the atlantic, indian and pacific oceans, all narrowing toward the north and, in the south, where the tetrahedral edges are lowest, uniting into a continuous belt around the globe. the actual form which lowthian green believes the earth to have reached is the hexakis-tetrahedron (see 7.575, fig. 20), and he supposes that all the faces are much rounded so that the departure from the spheroidal form is only slight. lowthian green’s theory was entirely neglected in england for many years, but was more favourably received in france. de lapparent seems to have been the first to recognise it as a probable hypothesis and at a later date both michel levy and marcel bertrand adopted it in a modified form. since the closing years of the last century a tetrahedral theory in some shape or other has been accepted by many writers in england, germany and america. modifications of the tetrahedral theory.—that the lithosphere shows some approximation to the tetrahedral form is matter of observation and not of theory. but that the approximation is only rough is indicated by the fact that the writers who support the theory do not all place the tetrahedron in the same position. michel levy’s tetrahedron, for example, is not the same as lowthian green’s. the question naturally arises whether so ill-defined an approach to the form is due to some general cause such as lowthian green imagined, or to the accidental concourse of a number of minor causes. if the form is due to a general cause, then, since three of the corners of the tetrahedron are placed symmetrically with respect to the axis of the earth, the three land-masses which represent them should have a similar geolog- ical history. in particular it might be expected that their coasts should show some uniformity of type. but, as has been pointed out by suess, the pacific and atlantic coasts differ fundamentally in character. the former runs parallel to the folds that have affected the surrounding land, and it is clear that the cause that produced the folds also determined the coast. the atlantic coast, on the other hand, in general cuts right across the folds that have affected the neighbouring land and must owe its origin to another cause. the difference is too great to be ac- counted for by later modifications of coasts which were originally of similar origin. further, if the differences are due to later modifications, the approximation to the tetrahedron should have been closer in the past than it is now. all the available evidence goes to show that the tetrahedron which fits best to the present distribution of land and sea will not fit at all to the dis- tributions in past times. on physical grounds objection has been raised that the tetra- hedron is not a figure of equilibrium fora rotating earth, and even a slight approximation to it cannot be retained. gregory has sug- gested that there may have been periods of collapse with an approach to the tetrahedral form, and in the intervening periods the spheroidal form was resumed. moreover, in these periods of collapse the tetrahedron may not always have been in the same position, and thus we get the different distributions of land and sea in the past. other writers also place the tetrahedron differ- ently for different periods, but the geological evidence in favour of these views is not convincing. suess’s theories —a great advance in our knowledge of the history of the globe was made by suess in his dus a nilitz der frde. we showed that there are large areas where even the oldest fossiliferous beds still lie horizontal. ifere the crust of the earth has been rigid, in the ordinary sense of the term, since cambrian times. it has broken, but it has never crumpled. between these areas lie broad belts in which the strata are often folded. here the crust has been weaker and has yielded to tangential pressures by crumpling and overthrusting. the crumpling was not a con- tinuous process. ‘there were well-defined periods of folding sep- arated by intervals during which the whole earth was free, or almost free, from such disturbances. during these intervals, however, fracturing of the resistant areas took place and large blocks sank; and there were also wide extensions or ‘‘ transgres- sions’ of the sea over the land. some oi these transgressions seem to have been nearly simultaneous throughout the world and the whole land-surface must have been greatly reduced. the most extensive of them, so far as positive evidence can be adduced, occurred in the middle and later parts of the cretaceous period. in the northern wemisphere there are three areas in which the cambrian beds remain unfolded. these are: (1) ‘ laurentia,” 178 which includes most of canada east of the rockies and probably stretched 1o the western is. of scotland, (2) the “ baltic shield ”’ and ‘‘ russian platform,” (3) “‘ angaraland,” which includes a great part, but not the whole, of eastern siberia. in all these areas, except upon the russian platform, no marine beds of mesozoic age have been found, and during the mesozoic era they were land. probably by that time laurentia and the baltic shield had been united by the folding of the intervening belt that took place at an earlier date. in the southern hemisphere cambrian fossils have only been found at one or two localities, but there are areas in which the oldest fossiliferous beds known show no folding. the most ex- tensive of these is ‘’ gondwana-land,” which includes the greater part of south america east of the andes, most of africa between the atlas and the mountains of the cape, arabia, syria and the peninsula of india. strictly speaking, perhaps, the whole of this region should not be called gondwana-land, for the creta- ceous sea extended far over it and even marine jurassic beds are found in places. but much the larger part was land throughout the mesozoic era. a similar remark applics to a great part of australia. between the mesozoic land-areas of the north and those of the south les a belt in which the mesozoic beds are for the most part marine. this is the “ tethys ”’ of suess. between the rigid masses of the north and those of the south the more yielding crust beneath the sea of tethys was crushed, and the great series of mountain ranges which run from west to east across the old world was elevated. tethys was much re- duced in size and is now represented chiefly by the mediterranean sea. about the same time laurentia and gondwana-land broke up, large portions sinking beneath the sea, and thus the present atlantic came into existence. such are some of the more important events in the history of the globe according to the researches of suess, and it is im- possible here to enter into further detail or examine the evidence more closely. the pacific appears to have been sea throughout the mesozoic era at least, but nevertheless it has functioned as a rigid area. suess imagines that the interior of the earth is contracting and the crust settling down upon it. the settling is accomplished partly by the fracturing of the more rigid portions, partly by the crumpling of the more yielding parts between them. sometimes one portion of the crust is pushed over another and thus there must have been changes in the relative positions of different points upon the surface, but in comparison with the size of the globe these changes are small. the wegener ivpothesis—wegencr introduces an entirely different conception, according to which the relative positions even of whole continents have altered greatly in quite recent times. the idea is not altogether new, for schwarz had already suggested that africa and south america were once nearer to each other; and other writers, on less solid ground, have made somewhat similar suggestions. but wegener goes further than his predecessors and brings forward a much greater body of evidence. sial, sima and nife——it has long been known to geologists that the visible part of the earth’s crust consists chieily of the eghter and more aci«l rocks, and beneath this it has been commonly sup- posed that there lies a layer of the denser and more basic rocks. the interior core must be denser still. in formulating these ideas suess proposed the names sal, sima and nife for the three con- centric regions respectively. the term sal is now usually replaced by sial, and with this modification suess’s nomenclature is widely adopted. it has generally been supposed that the sial covers the whole globe. it may be thinner under the oceans and thicker on the continents, but it is present everywhere. wegener beheves that the sial is discontinuous. the tloor of the ocean is formed of sima and the continents are sheets of sial floating in the sima. because the sial is of smaller density its surface rises above the surface of the sima. the sima is not liquid in the ordinary sense of the word; but there is no perfect solid, and the sheets of sial, supposed to be about 100 km. thick, could not lie upon the sima without the force of gravity pressing them in until they floated geology: new theories like a cork in water. this is quite in accordance with the ob- servations of geodesists, which indicate that the earth is very nearly in a state of isostatic equilibrium. it was long ago shown by pratt that there must be an excess of density beneath the oceans and a deficiency beneath the elevated regions; and airy suggested that all elevated masses must be supported by a downward protuberance of the lighter rock of which they are formed, into a denser layer below—in fact by flotation. it should be noted, however, that all the observations are equally consistent with the notion of a con- tinuous sial thinner in oceanic areas and thicker in elevated re- gions, the downward extension into the sima bearing such a relation to the protruding portion that the whole is in hydro- static equilibrium. wegener’s view is that the sheets of sial are not only separate and floating in the sima but also moving laterally, and that their positions relatively to one another have altered in the past and are altering still. there are certain forces arising from the rotation of the carth and the attraction of the moon which would tend to cause lateral movements. whether they are sufficient to produce such movement through so resistant a medium as the sima is open to question; but it is conceivable that they may be. wegener says that if we take the edge of the continental shelf as the edge of the sial sheets and allow for the effects of tertiary folding, the pieces of sial can be fitted together into one con- tinuous sheet covering about a third of the globe. this, he thinks, was the condition at the close of the palaeozoic era. he brings africa and south america together and both into contact with the antarctic continent. australia also is placed against antarctica, and madagascar and the end of an elongated india are wedged between africa and australia. in the north, green- jand, canada and north-western europe come together but a wide gap is left between the rest of north america and the continents of europe and africa. criticisms of the theory.—there is far too much scope for the imagination in this process. wegener docs not follow the edge of the continental shelf with any precision, and the allowance that he makes for the tertiary folding is purely fanciful. morcover, in his fitting he has introduced great distortion. if we keep the masses rigid and fit south america into the gulf of guinea, canada will not come within 1,000 m. of the position it occupies in his scheme. indeed, if his theory rested solely on the evidence of fitting, it might be dismissed at once. but there is much stronger evidence than this. he says that with his fitting the ecological structures on the eastern side of the atlantic become the direct continuation of the structures on the western side. the argument is weakened by the great amount of distortion that he has introduced to make them fit, it is almost entirely destroyed by the real facts. several of his statements are quite at variance with those of the actual observers, and others rest on a very imperfect basis. it is in the southern hemisphere that the geological evidence for a former union of the continental masses is strongest, and it is perhaps significant that the nearest approach to a fit is that between africa and south america. ‘the close similarity between the rocks and fossils of these two areas has led most geologists to assume a former land connection. both regions are char- acterised by the occurrence of the fossil glossepteris flora, which differs considerably from the contemporancous flora of europe. the same flora has also been found in india and australia. all these are brought together in wegener’s scheme and the sim- ilarity in rocks and fossils is at once explained. but the argu- ment is considerably weakened by the fact that the glossepterts flora is also found in siberia and northern russia, which in his reconstruction become farther away from the presumed home of glossopterts than they are at present. in view of our very im- perfect knowledge of the geology of asia, and the discovery of the glossopteris flora in a few other asiatic localities, it seems prob- able that it had a wide distribution and was far from confined to the regions that wegener brings together. associated with the glossopieris beds in south america, africa, glometry india and australia there is a boulder bed containing glaciated boulders, which has long been a puzzle to geologists. on the most favourable supposition with regard to the position of the south pole the ice that carried the boulders must have extended nearly to the tropics, and it seems improbable that any glacial period could have reached the required intensity. in wegener’s re- construction of the period all these areas come together and would lie within a reasonable distance of a suitably placed pole. but here again there are difficulties. a similar boulder bed, also associated with beds containing a glossoptcris flora, has been found in afghanistan, which, with wegener’s fitting, must have been within thirty degrees of the equator when the other areas were centred about the pole. the afghanistan boulder bed was laid down near sea-level, for it is associated with an alternation of marine and terrestrial deposits. in general it may be said that wegener’s theory removes cer- tain difficulties, but in removing them it introduces others of equal magnitude. by far the most suggestive point in his favour is the resemblance between the african and south american rocks and fossils. even here coleman, who has ex- amined the south american boulder bed, states that the ice which deposited it reached the sea on the eastern side of south america. joly and radio-activity.—joly’s theory is based on the effects of racdio-activity. like wegener he thinks that the continents are shects of sial floating in the sima, which forms the floor of the ocean. relative displacements of the continents may have taken place, but only at certain periods. the sial and the sima are both radio-active and the radio- active elements in them are continually generating heat. unless this heat can escape the temperature must rise. he melting point of the sial rocks is known to be much higher than that of the sima, which is basaltic in composition. suppose that at a certain period the whole of the sial is solid and the sima also is solid down to a considerable depth. heat can only escape by the slow process of conduction. beneath the sial there will be no escape from the sima, because the base of the sial itself, owing to its own radio-activity, must be nearly at the melting point of sima. since the escape of heat is so slow the temperature rises and the sima melts to within some 20 m. of the sutface. tidal move- ments in the molten sima acting upon the downward projections of the sial move the whole crust so that the local accumulations of heat originally formed beneath the sial come to lie under the thin layer of solid sima beneath the ocean. this is quickly thinned still further, and molten sima escapes through fractures. the loss of heat now becomes more rapid and an era of cooling and consolidation begins. we must suppose, then, that there are periods during which the sima layer melts more or less completely and periods during which it solidifies down to a considerable depth. it has been shown ex- perimentally that basalt, and most other rocks that have been tried, expand on melting and decrease in density. as the sima melts it expands, and therefore the general level of the earth’s surface is raised. but because the density of the sima is de- creased the masses of sial which are floating in it sink more deeply into the layer. ‘their surfaces, owing to the general rise, become farther from the earth’s centre, but, relatively to the surface of the sima they are depressed. in this way the wide- spread transgressions characteristic of certain geological periods are explained. as the sima cools again and becomes denser the surface falls, but the sial masses rise relatively to the sima. when the sima begins to cool, the crust upon it, if not already complete, is soon completed by the consolidation of molten ma- terial in its fissures. together with the sial masses it now covers the whole globe. but the sima continues to contract further, and the covering becomes too large. and now begins a period of folding and other earth movements to enable the crust to descend with the sima. it is not possible here to enter further into the consequences of the theory. its great merit, from the geological point of view is that it seems to offer a clear explanation of the most remarkable iony 179 features in the history of the globe, the periodicity of earth- folding and sea-transgressions and their widespread character. conclusion.—in view of our ignorance of what goes on in by far the greater part of the interior of the globe it remains im- probable that any theory founded on examination of a thin ex- ternal skin can be wholly truc. the old hypothesis of a contract- ing core fits the observed facts of geology as well as any other. it has been urged that the possible amount of contraction is in- sufficient, but on this point it is unsafe to dogmatise. recent astronomical observations show that the companion star of sirius consists of gas and yet is denser than any material upon the earth’s surface. a discovery so unexpected leads us to revise our ideas and, though the conditions are very different, renews interest in the suggestion of arrhenius that the interior of the earth is gaseous. brbliography.—w. vowthian green, veseiges of the molten globe (london, 1875); fe. suess, das amelits der erde (1883-1909)— french (1897) and english (1904) translations have been published: a. wegener, die entstehung der kentinente und oseane (1920)— an [english translation appeared in 1924; j. joly, “the movement of the ifarth’s surface crust,” phil. mag. (1923), also radio-activity and the surface [istory of the hurth, halley lecture (1924) and phe surface history of the earth (oxford, 1925); h. jcffreys, the earth (1924) is of special interest from the point of view of geo- physics in general and lends support to the contraction hypothesis. (pad",
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