430 ORIGIN OF THE OOLITE AND LIAS. [Ch. XXI. 



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

 ine, 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 neighbor- 

 ing 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, arenaceous 

 matter replaces the clay ; because it commonly happens that the finer 

 sediment, being first drifted farthest from coasts, is subsequently over- 

 spread by coarse sand, after the sea has grown shallower, or when 

 the land increasing in extent, whether by upheaval or by sediment 

 filling up parts of the sea, has approached nearer to the spots occupied 

 by fine mud. 



In order to account for another great formation, like the Oxford 

 clay, again covering one of coral limestone, we must suppose a sink- 

 ing down like that which is now taking place in some existing 

 regions of coral between Australia and South America. The oc- 

 currence of subsidences, on so vast a scale, may have caused the bed 

 of the ocean and the adjoining land, throughout great parts of the 

 European area, to assume a shape favorable 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 descend- 

 ing 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 

 of thickness, may have required centuries for its completion, during 

 which certain species of organic beings disappeared from the earth, 

 and others were introduced in their place ; so that, in each set of strata, 

 from the Lias to the Upper Oolite, some peculiar and characteristic 

 fossils were imbedded. 



