Lower 

 Siluriaa 



132 



f Upper Llandovery beds 

 Lower Llandovery beds 

 Hirnant Limestone 

 Caradoc 

 Bala Limestone 

 Caradoc 

 Llandeilo 



\ 



j Llandovery series 



Caradoc series 



After a re-examination of the Shropshire rocks it was 

 , ascertained by M. Avelijste, and corroborated by my friend 

 Professor Eamsay,* that the beds with Pentamerus IcBvis in the 

 Llandovery series He quite nnconformably on the true Caradoc 

 Sandstone, and that there is a great physical break between the 

 Upper Llandovery beds and the Silurian strata below. "In 

 Shropshire the unconformity is visible. The Upper Llandovery 

 beds on the banks of the Onny lie on the higher part of the 

 Caradoc Sandstone, (the Bala beds,) and as they strike northward 

 gradually overlap the higher strata, till, on the banks of the 

 Severn, near BuUdwas Abbey, they rest on the lower beds of the 

 same formation. A few miles from Wenlock Edge they lie on 

 the nearly vertical edges of the Cambrian or Longmynd rocks, and 

 also on Lingula and Llandeilo flags between Church Stretton 

 and Chirbury. In South Wales, between Builth and Newbridge, 

 they lie equally nnconformably on the Llandeilo beds." The 

 break or unconformity between the Upper and Lower Silurian 

 rocks in South Wales is therefore complete, and affords physical 

 evidence of the vast period of time that must have elapsed during 

 the deposition of these formations. This break likewise accounts 

 for the difference observed between the specific forms of the 

 Fossils contained in the upper and lower divisions. 



In Bohemia M. Barkande has divided the Siliu-ian period into 

 eight stages — of which four appei*tain to the lower and four to the 

 upper division. In the lower stages, A and B are non-fossiliferous, 

 C and D fossiliferous ; the upper stages, E, F, G, H, are all 

 fossiliferous. In other European countries, where Silurian 

 rocks are found, they admit of a like sub-division. In fact, 



Memoirs of the Geological Survey, Geology of North Wales, Vol. 3, page 5, 1866. 



