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LLANDEILO GROUP

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Originally appearing in Volume V16, Page 828 of the 1911 Encyclopedia Britannica.
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LLANDEILO See also:

GROUP , in See also:geology, the See also:middle subdivision of the See also:British Ordovician rocks. It was first described and named by See also:Sir. R. I. See also:Murchison from the neighbourhood of Llandeilo in See also:Carmarthenshire. In the type See also:area it consists of a See also:series of slaty rocks, shales, calcareous flagstones and sandstones; the calcareous middle portion is sometimes termed the " Llandeilo See also:limestone "; and in the upper portion volcanic rocks are intercalated. A remarkable feature in the See also:history of the Llandeilo rocks in See also:Britain, more especially in See also:North See also:Wales and See also:Cumberland,was the outbreak of volcanic See also:action; vast piles of Llandeilo See also:lava and ashes See also:form such hills as Cader Idris, and the Arenigs in Wales, and Helvellyn and See also:Scafell in See also:Westmorland and Cumberland. The series is also found at See also:Builth and in See also:Pembroke-See also:shire. The See also:average thickness in Wales is about 2000 ft. The group is usually divided in this area into three sub-divisions. In the Corndon See also:district of See also:Shropshire the See also:Middleton Series represents the Llandeilo group; it includes, in descending See also:order, the Rorrington See also:black shales, the Meadowtown limestones and flags, and the western grits and shales. In the See also:Lake District the See also:great volcanic series of Borrowdale, See also:green slates and porphyries, 8000 to 9000 ft. in thickness, lies on this See also:horizon; and in the See also:Cross See also:Fell area the Milburn beds of the Skiddaw slates (see ARENIG) appear to be of the same See also:age.

In See also:

Scotland the Llandeilo group is represented by the Glenkiln shales, black shales and yellowish mudstones with radiolarian cherts and volcanic tuffs; by the See also:Barr Series, including the Benan conglomerates, Stinchar See also:lime-See also:stone and Kirkland sandstones; and by the Glenapp conglomerates and Tappins'mudstones and grits See also:south of Stinchar. Graptolitic shales, similar to those of See also:southern Scotland, are traceable into the north-See also:east of See also:Ireland. The fossils of the Llandeilo group include numerous See also:graptolites, Coenograplus gracilis being taken as the zonal fossil of the upper portion, Didymograptus Murchisoni of the See also:lower. Other forms are Climacograptus Scharenbergi and Diplograplus foliaceus. Many See also: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, See also:Bellerophon, Murchisonia among the Gasteropods; Conularia and Hyolithes among the Pteropods; the Cephalopods are represented by Orthoceras and Cyrtoceras. The green roofing slates and See also:plumbago (See also:graphite) of the Lake District are obtained from this group of rocks. (see ORDOVICIAN).

End of Article: LLANDEILO GROUP

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