CONTEMPORANEITY OF STRATA. 51 



Foraminfera, some of which are specifically identical with 

 Cretaceous forms, whilst White Chalk is known to be very 

 largely composed of the ddbris of these minute organisms. 3. 

 The ooze contains siliceous sponges, in many respects com- 

 parable to the sponges which are so characteristic of the 

 Cretaceous period. 4. The ooze contains Echinoderms, espe- 

 cially Sea-urchins and Crinoids, such as abounded in the 

 Chalk period ; whilst one of the latter is related to a Creta- 

 ceous type hitherto believed to be extinct. 



On the other hand, as pointed out by Sir Charles Lyell, 

 Prof. Prestwich, and other observers, the differences between 

 the Atlantic ooze and the Chalk are, to say the least of it, 

 quite as weighty as the resemblances, if not more so. Chalk 

 is composed of from eighty to as much as ninety-nine per 

 cent of carbonate of lime, and has therefore a very small pn>- 

 portion of any siliceous or aluminous impurity. Secondly, 

 the occurrence of identical species of Foraminifera in the 

 two formations amounts to very little ; for it is well known 

 that such lowly organised forms of life have an extraordinary 

 power of persistence, surviving geological changes which are 

 fatal to higher organisms. Moreover, it seems certain that 

 the Foraminifera of the Atlantic ooze are principally derived 

 from the surfaee-waters of the ocean, so that they prove noth- 

 ing as to the depth at which the ooze was deposited. Thirdly, 

 Dr Gwyn Jeffreys, one of the highest of authorities upon the 

 Mollusca, has shown that the Molluscan fauna of the Chalk 

 is essentially a shalloiv-water fauna, and certainly cannot be 

 supposed to have inhabited any very great depth. Lastly, 

 the most characteristic of the Chalk fossils, such as the various 

 forms of Cephalopoda and Bivalve Molluscs, are entirely want- 

 ing in the Atlantic ooze. 



Prof. Prestwich concludes that although it is probably true 

 that " some considerable portion of the deep sea-bed of the 

 mid-Atlantic has continued submerged since the period of 

 our Chalk, and although the more adaptable forms of life 

 may have been transmitted in unbroken succession through 

 this channel, the immigration of other and more recent 

 faunas may have so modified the old population that the 

 original Chalk element is of no more importance than is the 



