230 PICTURESQUE SKETCHES 



the oolites, so far as the Invertebrata are concerned, 

 while the same large marine saurians seem to have 

 been continued. This may readily be understood, if 

 we suppose, that, in the interval between the deposit 

 of the uppermost marine oolites and the lower green- 

 sand there was an extensive tract of land at some 

 distance from the districts examined. I know of no 

 other hypothesis by which the facts can be explained ; 

 and this view is extremely probable in itself, and 

 answers the conditions of the problem. To this view 

 I shall have occasion again to recur. 



When, however, towards the close of the deposits 

 of greensand, the sea began to deepen, the character 

 of the fauna also changed, and we find gradually 

 fewer indications of the neighbourhood of land, either 

 by fragments of land fossils, or by any indications of 

 those molluscous animals which prefer for their habita- 

 tion the shallow water near a coast-line. The animals 

 and other remains of greatest interest that we shall 

 have to consider as characterising the chalk are thus 

 either free-swimming, or their nearest analogues are 

 known to inhabit deep water. They include sponges, 

 the minute animals called Foraminifera., those still 

 more minute microscopical animalcules called Infu- 

 soria, various forms of encrinites and other Eadiata, 

 cephalopodous and other mollusks, a few fishes, and 

 some, but still fewer, reptiles. Most of the species 

 are peculiar to the formation, but there is a manifest 

 approximation to the existing forms of animals, not 

 only in those of low organization, but also in the 

 fishes. 



In the most ancient seas we find abundant exam- 

 ples of the work, of those singular beings, which, in 



