72 Llandeilo and Caradoc Beds. 



Cader Idris, Aran Mowddwy, Arenig, and the Moelwyns, 

 resting on the lava beds and ashes, and overlaid on the 

 east by Upper Silurian strata, fig. 57, p. 304. They 

 also form, with igneous rocks, the larger part of the 

 Berwyn mountains, and with the Arenig slates the 

 whole of the ground between the Stiper Stones and the 

 Upper Silurian rocks of Chirbury and Montgomery, fig. 

 13, p. 59. The typical Caradoc Sandstone, crossing 

 the strike, ranges between Church Stretton and Caer 

 Caradoc, from whence it stretches in a broad band 

 northward towards the Wrekin, and southward to 

 Corston. The greater part of South Wales is formed 

 of slates and grits of Llandeilo and Caradoc age, lying 

 west and north of the Upper Silurian and Old Red 

 Sandstone strata, and the same formations, associated 

 with volcanic rocks, rise like an island surrounded by 

 Upper Silurian strata, in the country between Builth 

 and Llandegley in Radnorshire. 



In South Wales, where they were first described by 

 Murchison, the Llandeilo beds consist of sandy cal- 

 careous flags, black slaty rocks, and beds of grit and 

 sandstone. A few beds of limestone occur in them in 

 Carmarthenshire, at Llandeilo, and in Pembrokeshire 

 near Narberth ; and the Bala limestone is found 

 higher in the series in the Caradoc or Bala beds of 

 Merionethshire. They are often highly fossiliferous. 

 There is a much larger development of fossils in the 

 Llandeilo flags than in the pre-existing Silurian strata. 

 The Trilobites of the Llandeilo beds are mostly peculiar 

 to it, and the genera JEglina, Barrandia, and Ogygia 

 are very common, Ogygia Buchii being especially cha- 

 racteristic. Viewed as a whole, however, the Llandeilo 

 beds, as already stated, pass insensibly into, and have 

 many genera and species in common with the Caradoc 



