LLANDEILO GROUP

llandeilo group, in geology, the middle subdivision of the british ordovician rocks. it was first described and named by sir. r. i. murchison from the neighbourhood of llandeilo in carmarthenshire. in the type area it consists of a series of slaty rocks, shales, calcareous flagstones and sandstones; the calcareous middle portion is sometimes termed the "llandeilo limestone"; and in the upper portion volcanic rocks are intercalated. a remarkable feature in the history of the llandeilo rocks in britain, more especially in north wales and cumberland, was the outbreak of volcanic action; vast piles of llandeilo lava and ashes form such hills as cader idris, and the arenigs in wales, and helvellyn and scafell in westmorland and cumberland. the series is also found at builth and in pembrokeshire. the average thickness in wales is about 2000 ft. the group is usually divided in this area into three subdivisions. in the corndon district of shropshire the _middleton series_ represents the llandeilo group; it includes, in descending order, the rorrington black shales, the _meadowtown limestones_ and flags, and the western grits and shales. in the lake district the great _volcanic series of borrowdale_, green slates and porphyries, 8000 to 9000 ft. in thickness, lies on this horizon; and in the cross fell area the _milburn beds_ of the skiddaw slates (see arenig) appear to be of the same age. in scotland the llandeilo group is represented by the _glenkiln shales_, black shales and yellowish mudstones with radiolarian cherts and volcanic tuffs; by the _barr series_, including the benan conglomerates, stinchar limestone and kirkland sandstones; and by the glenapp conglomerates and tappins mudstones and grits south of stinchar. graptolitic shales, similar to those of southern scotland, are traceable into the north-east of ireland. the fossils of the llandeilo group include numerous graptolites, _coenograptus gracilis_ being taken as the zonal fossil of the upper portion, _didymograptus murchisoni_ of the lower. other forms are _climacograptus scharenbergi_ and _diplograptus foliaceus_. many trilobites are found in these rocks, e.g. _ogygia buchi_, _asaphus tyrannus_, _calymene cambrensis_, _cheirurus sedgwickii_. among the brachiopods are _crania_, _leptaena_, _lingula_, _strophomena_; _cardiola_ and _modiolopsis_ occur among the pelecypods; _euomphalus_, _bellerophon_, _murchisonia_ among the gasteropods; _conularia_ and _hyolithes_ among the pteropods; the cephalopods are represented by _orthoceras_ and _cyrtoceras_. the green roofing slates and plumbago (graphite) of the lake district are obtained from this group of rocks, (see ordovician).