PART 11. CHAPTER XVIII. 



233 



Origin of the Oolite and Lias. 



pretty uniform character, for distances of from 400 to 600 miies 

 from east to west and north to south. 



According to M. Thirria, the entire ooHtic group in the depart- 

 ment of the Haute Saone, in France, may be equal in thickness 

 to that of England ; but the importance of the argillaceous divi- 

 sions is in the inverse ratio to that which they exhibit in Eng- 

 land, 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 argillaceous sediment, brought by oceanic currents, 

 which may have communicated 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 submerged, or 

 because the current is deflected in another direction by the 

 altered shape of the bed of the ocean and neighbouring dry 

 land. By such changes the water becomes once more clear and 

 fit for the growth of stony zoophytes. Calcareous sand is then 

 formed from comminuted shell and coral, or, in some cases, are- 

 naceous 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 sea has grown 

 shallower, or when the land, increasing in extent, has approached 

 nearer to the spots first occupied by fine mud. 



In order to account for another great formation, like the Ox- 

 ford clay, again covering one of coral limestone, we must sup- 

 pose a sinking down like that which is now taking place in some 

 existing regions of coral between Australia and South America. f 

 The occurrence of subsidences, on so vast a scale, may again 

 have caused the bed of the ocean and the adjoining land through- 

 out the European area, to assume a shape favourable to the 

 deposition of another set of clayey strata ; and this change may 

 have been succeeded by a series of events analogous to that 

 already explained, and these again by a third series in similar 

 order. Both the ascending and descending movements may 

 have been extremely slow, like those now going on in the 

 Pacific ; and the growth of every stratum of coral, a few feet 

 in thickness, may have required centuries for its completion, 

 during which certain species of organic beings may have disap- 

 peared from the earth, and others have been introduced in their 



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



t See Darwin, chap. xxii. 



