232 



LYELL'S ELEMENTS OF GEOLOGY. 



Fossils of the Lias Origin of the Oolite and Lias. 



accumulated slowly in the sea of the lias, some being formed 

 chiefly of one description of shell, such as ammonites, others of 

 gryphites. 



Fossil plants. — Among the vegetable remains of the Lias, 

 several species of Zamia have been 

 Fig. 230. found at L^'^me Regis, and the re- 



mains of coniferous plants at Whitby. 

 Fragments of wood are common, and 

 often converted into argillaceous lime- 

 stone. That some of this wood, 

 though now petrified, was soft when 

 it first lay at the bottom of the sea, 

 is shown by a specimen now in the museum of the Geological 

 Society, (see Fig. 230.) which has the form of an ammonite 

 indented on its surface. 



Origin of the Oolite and Lias. — If we now endeavour to 

 restore, in imagination, the ancient condition of the European 

 area at the period of the Oolite and Lias, we must conceive a 

 sea in which the growth of coral reefs and shelly limestones, 

 after proceeding without interruption for ages, was liable to be 

 stopped suddenly by the deposition of clayey sediment. Then, 

 again, the argillaceous matter, devoid of corals, was deposited 

 for ages, and attained a thickness of hundreds of feet, until ano- 

 ther period arrived when the same space was again occupied by 

 calcareous sand, or solid rocks of shell and coral, to be again 

 succeeded by the recurrence of another period of argillaceous 

 deposition. Mr. Conybeare has remarked of the entire group of 

 Oolite and Lias, that it consists of repeated alternations of clay, 

 sandstone, and limestone, following each other in the same order. 

 Thus the clays of the lias are followed by the sands of the infe- 

 rior oolite, and these again by shelly and coralline limestone, 

 (Bath oolite, &c. ;) so, in the middle oolite, the Oxford clay is 

 followed by calcareous grit and " coral rag ;" lastly, in the 

 upper oolite the Kimmeridge clay is followed by the Weymouth 

 sands and the Portland limestone.* The clay beds, however, as 

 Mr. De la Beche remarks, can be followed over larger areas 

 than the sands or sandstones. f It should also be remembered, 

 that while the oolitic system becomes arenaceous, and resembles 

 a coal-field in Yorkshire, it assumes, in the Alps, an almost 

 purely calcareous form, the sands and clays being omitted ; and 

 even in the intervening tracts, it is more complicated and vari- 

 able than appears in ordinary descriptions. Nevertheless, some 

 of the clays and intervening limestones do, in reality, retain a 



* Con. and Phil. p. 166. 



t Geol, Researches, p. 337. 



