224 



LYELL'S ELEMENTS OF GEOLOGY. 



CHAPTER XVIIL 



OOLITE AND LIAS — Continued. 



Mineral character of Lias — Name of Gryphite limestone — Fossil fish — Ich- 

 thyodorulites — Reptiles of the Lias — Ichthyosaur and Plesiosaur — Newly disco- 

 vered marine Reptile of the Galapagos Islands — Sudden death and burial of 

 fossil animals in Lias — Origin of the Oolite and Lias, and of alternating calcare- 

 ous and argillaceous formations — Physical geography — Vales of clay — Hills and 

 escarpments of limestone. 



Lias. — The English provincial name of Lias has been very 

 generally adopted for a formation of argillaceous limestone, marl, 

 and clay, which forms the base of the oolite, and is classed by 

 many geologists as part of that group. They pass, indeed, into 

 each other in some places, as near Bath, a sandy marl called 

 the marlstone of the Lias being interposed, and partaking of the 

 mineral characters of the upper lias and inferior oolite. These 

 last mentioned divisions have also some fossils in common, such 

 as the Avicula incequivalvis (Fig. 220.) 

 Nevertheless the Lias may be traced through- 

 out a great part of Europe as a separate and 

 independent group, of considerable thickness, 

 varying from 500 to 1000 feet, containing 

 many peculiar fossils, and having a very 

 uniform lithological aspect. Although usu- 

 .avicuia inmqnivaivis, ^Uy Conformable to the oolite, it is sometimes, 

 Sow. as in the Jura, unconformable. Thus, in 



the environs of Lons-le-Saulnier, for instance, the strata of lias 

 are inclined at an angle of about 45^, while the incumbent ooli- 

 tic marls are horizontal. 



The peculiar aspect which is most characteristic of the Lias 

 in England, France, and Germany, is an alternation of thin 

 beds of limestone, with a light brown weathered surface, sepa- 

 rated by dark-coloured narrow argillaceous partings, so that the 

 quarries of this rock, at a distance, assume a striped and riband- 

 like appearance.* 



Although the prevailing colour of the limestone of this form- 

 ation is blue, yet some beds of the lower lias are of a yellowish 



* Conyb. and Phil. p. 261. 



