Murchison s Silurian System. 



229 



in some places worked for troughs, tomb-stones and building 

 purposes. 



The calcareous grit forming the lowest bed of the Cara- 

 doc formation, contains 



Terebratula (orthis) anomala, 



Pentamerus oblongus, and the plumose Coral, 

 Calamopora fibrosa, Goldf. 



The Caradoc sandstone reposes on deep reddish purple 

 sandstones, with streaks of dirty yellowish-green, in beds from 

 six inches to two and three feet. Where these beds are 

 exposed, Mr. Murchison found in their superficial strata 

 casts of 



Orthis flabellulum, 



O. vespertilio, 



Terebratula unguis, 

 These beds are dissimilar to any of the overlying strata 

 from their red colour and intermixture with clay and marl. 

 In other situations, the section of these lower Silurian rocks 

 present 



" Grits and coarse sandstone of brown and yellow colour. Pure white 

 grained sandstone, consisting of grains of sand imbedded in a matrix of 

 felspars, which decomposing, give the whole a freckled appearance. 

 Yellowish sandstone with ferruginous streaks, deep red sandstone, and 

 whitish gritty sandstone." 



These sandstones are in places cut through and thrown 

 into vertical position by eruptive trap rocks. In other situ- 

 ations, quartzose pebbles are held together by a ferruginous 

 cement. This formation is liable to be mistaken for old, or 

 even new red sandstone ; but its best distinction, says Mr. 

 Murchison, consists in its position below the upper Silurian 

 rocks, and in its organic remains, which are different from 

 those of any formations that overlie it. 



The minerals which occur in the lower Silurian rocks ? <are 

 green carbonate of copper, and other copper ores, thin 

 seams of galena with associated crystals of blende ; and 



2 H 



