Ch XXL] OEIGIN OF THE OOLITE AND LIAS. 329 



coral reefs and shelly limestones, after proceeding without interruptioii 

 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 

 another period arrived when the same space was again occupied by cal- 

 careous sand, or solid rocks of shell and coral, to be again succeeded by 

 the recurrence of another period of argillaceous deposition. Mr. Cony- 

 beare 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 inferior oolite, and these again by shelly and coralline lime- 

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

 lowed by calcareous grit and " coral rag ;" lastly, in the upper oolite, the 

 Kimmeridge clay is followed by the Portland sand and limestone."'^ The 

 clay beds, however, as Sir H. De la Beche remarks, can be followed 

 over larger areas than the sands or sandstones.f It should also be re- 

 membered that while the oolitic system becomes arenaceous, and resem- 

 bles 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 variable than appears in 

 ordinary descriptions. Nevertheless, some of the clays and intervening 

 limestones do, in reality, retain a pretty uniform character, for distances 

 of from 400 to 600 miles from east to west and north to south. 



According to M. Thirria, the entire oolitic group in the department of 

 the Haute Saone, in France, may be equal in thickness to that of Eng- 

 land ; but the importance of the argillaceous divisions is in the inverse 

 ratio to that which they exhibit in England, where they are about equal 

 to twice the thickness of the limestones, whereas, in the part of France 

 alluded to, they reach only about a third of that thickness.]; In the 

 Jura the clays are still thinner ; and in the Alps they thin out and 

 almost vanish. 



In order to account for such a succession of events, we may imagine, 

 first, the bed of the ocean to be the receptacle for ages of fine argilla- 

 ceous sediment, brought by oceanic currents, which may have communi- 

 cated with rivers, or with part of the sea near a wasting coast. This 

 mud ceases, at length, to be conveyed to the same region, either because 

 the land which had previously suffered denudation is depressed and sub- 

 merged, or because the current is deflected in another direction by the 

 altered shape of the bed of the ocean and neighboring dry land. By 

 such changes the water becomes once more clear and fit for the growtli 

 of stony zoophytes. Calcareous sand is then formed from comminuted 

 shell and coral, or, in some cases, arenaceous matter replaces the clay ; 

 because it commonly happens that the finer sediment, being first drifted 

 farthest from coasts, is subsequently overspread by coarse sand, after the 



* Con. and Phil. p. 166. f Geol. Researches, p. 331. 



} Burat's D'Aubuisson, torn, ii. p. 456. 



