SECONDARY LIMESTONES. 241 



rarely, oolithic. Occasionally, it is magnesian, or bi- 

 tuminous, and, in some places, resembles the English 

 magnesian limestone. It presents all the usual co- 

 lours ; whitish, greys of various hues, and yellows, 

 tending to reds and browns. Nodules of cherty flint 

 and of pyrites occur in it, as do beds of marl. The 

 fossils are, chiefly, encrinites, pectcn, tercbratnla, am- 

 monites, mytilus, turbo, astrea, echinus., tnhipores, 

 millepores, and alcyonia. 



The next principal deposit of limestone upwards, is 

 the Lias of England, separated from the last, as I 

 have just remarked, by the qnadersandstein, on the 

 continent, but, with us, following the red marl. If it 

 has sometimes been associated with the succeeding 

 calcareous series, the oolithe, it seems most conve- 

 nient to consider them as separate deposits. The 

 total series is, properly, a collection of argillaceous 

 beds containing limestone ; the calcareous strata pre- 

 dominating in the inferior parts. These are com- 

 monly thin, and separated by clay and shale ; being, 

 in the upper portions, often interspersed through a 

 blue marl. Such at least is a general character for 

 "the English examples ; but, on the continent, it pre- 

 sents other variations which I need not detail. This 

 limestone is generally distinguished by its very argil- 

 laceous character and aspect ; being most commonly 

 grey, of various tones, passing to white; while, from 

 containing large portions of iron and clay, it is used 

 for water cements. 



If I already noticed some important inferences re- 

 specting former conditions of the Earth, to be derived 

 from certain organic fossils in this series, I may here 

 again suggest some striking resemblances between the 

 Lias and the plastic clay, or the deposits immediately 

 succeeding the chalk, as opening to observations 

 * VOL. a. R 



