Ch. XXI.] 



MINERAL CHARACTER OF THE LIAS. 



317 



CHAPTER XXL 



jurassic group — continued, lias. 



Mineral character of Lias — Name of Gryphite limestone — Fossil shells and fish — 

 Eadiata — Iehthyodorulites — Reptiles of the Lias — Ichthyosaur and Plesiosaur 

 — Marine Reptile of the Galapagos Islands — Sudden destruction and burial of 

 fossil animals in Lias — Fluvio-marine beds in Gloucestershire, and insect lime- 

 stone — Fossil plants — Origin of the Oolite and Lias, and of alternating calca- 

 reous and argillaceous formations — Oolitic coal-field of Virginia, in the United 

 States. 



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

 adopted for a formation of argillaceons 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 lias and the inferior oolite. 

 These last-mentioned divisions have also some fossils in common, such as 

 the Avicula incequivalvis (fig. 398). Nevertheless, the Lias may be 



Fig. 399. 



Fig 398. 



Avicula inmquivalvis, Sow. 

 Lower Oolite. 



Avicula cygnipes, Phil. 

 Marlstone, Gloucestershire ; Lias, Yorkshire. 



traced throughout a great part of Europe as a separate and independent 

 group, of considerable thickness, varying from 500 to 1000 feet, contain- 

 ing many peculiar fossils, and having a very uniform lithological aspect. 

 Although usually conformable to the oolite, it is sometimes, as in the 

 Jura, unconformable. In the environs of Lons-le-Saulnier, for instance, 

 in the department of Jura, the strata of lias are inclined at an angle of 

 about 45°, while the incumbent oolitic marls are horizontal. 



The peculiar aspect which is most characteristic of the Lias in Eng- 

 land, France, and Germany, is an alternation of thin beds of blue or gray 

 limestone, having a surface which becomes light-brown when weathered, 



