IN THE MUD OF THE LEVANT. 71 



thus exhibiting a stratum which is ahnost entirely 

 siliceous, with the exception of a few minute and 

 scattered calcareous organisms. It is possibly of 

 the age of our cretaceous rocks. 



Thus far we find that, with the exception of the 

 South American strata, the deposits have been 

 formed by agencies very similar to those still 

 operating in our seas, and on our sandy beaches. 

 That where any large amount of calcareous matter 

 is present, such as cannot be accounted for on 

 the supposition of detritus from more ancient 

 strata, there we usually find either Foraminifera 

 or disintegrated shell structures. Where we 

 have extensive siliceous strata, containing sili- 

 ceous organisms, but none of a calcareous charac- 

 ter, we have in all probability deposits in which 

 chemical agencies have effected great changes. 

 But to this subject we shall have to recur. Let 

 us now see how far down the geological scale 

 similar illustrations are to be found. 



Cretaceous Strata. — As before stated, several 

 observers have examined the Chalk rocks since 

 the first discovery of the fact, that they chiefly 

 consist of minute Polythalamia. This has been 

 especially done by M. Ehrenberg and M. Alcide 



