228 



Murchisoris Silurian System. 



THE LOWER SILURIAN ROCKS. 



Caradoc Sandstone. — Unlike the upper Silurian rocks, these 

 are composed essentially of sandstone of different colours, 

 with an occasional thin bed of impure sandy limestone. In 

 these beds, all traces of the Trilobites common to the upper 

 Silurian rocks are lost, and in place of them occurs Trinu- 

 cleus caractace, Murch., which belongs to a genus never 

 observed in the upper Silurian rocks. The upper group of 

 this division of Silurian rocks is described by Mr. Murchison 

 as thinly laminated sandy shale, only slightly micaceous, con- 

 taining layers of pipe clay and thin bands of impure sandy 

 limestone. The fossils characteristic of these beds are, 

 Productus sericeus, 

 Bellerophon bilobatus, 

 B. acutus, 

 Littorina striatella, 

 Orthis alternata, 

 — — callactis, 



canalis^ 



— — - pecten, 

 Avicula obliqua, 

 Arbicula granulata. 

 The upper beds of this formation dip beneath the Wen- 

 lock shale, and the lower strata graduate into flagstones. On 

 fracturing these flagstones, Mr. Murchison remarks, the 

 fossils stand out in neat casts covered with brown and yel- 

 low hydrate of iron, presenting a marked relief from the 

 dingy yellow olive green sandstone. 



The characteristic fossils in this group of flag strata are, 

 Orthis actonia, 

 O. grandis, 

 Trinucleus car act ace, 

 T. fimbriatus. 



This group of Caradoc sandstone, Mr. Murchison thinks, 

 is not less than four hundred feet thick. The flags are 



