238 GEOGRAPHICAL AXD GEOLOGICAL DISTRIBUTION. 



biological relationship is not restricted to the lower or microscopic 

 forms of life, but, on the contrary, manifests itself in almost as 

 marked a degree in some of the higher groups. Thus the sponges 

 of the Atlantic ooze appear to have been most closely related to 

 the ventriculite sponges of the Cretaceous period, and there is also 

 a predominance in both classes of deposits of the cidaroid type of 

 sea-urchin. 



As opposed to the biological relationship, on the other hand, it 

 has been pointed out that the physical constitution of the two 

 classes of deposits is very different. Thus, while Cretaceous chalk 

 almost invariably contains from ninety-four to ninety-nine per cent, 

 of carbonate of lime, and, consequently, at the utmost only about 

 six per cent, of foreign substances, analyses of Globigerina ooze 

 show it to contain only from forty-nine to eighty per cent., allow- 

 ing a very considerable percentage for impurities. Such a differ- 

 ence in chemical composition is certainly very striking, the more 

 especially as the composition of true chalk is very constant, and 

 appears to point to a mode of formation different in the two 

 cases. The correspondence in chemical composition existing be- 

 tween true chalk and the Oahu chalk (coral debris) of the Sand- 

 wich Islands has suggested to Mr. Wallace the notion that not 

 improbably both deposits have much the same formation in other 

 words, that the chalk was deposited in a comparatively shallow 

 sea, probably not exceeding one thousand fathoms in depth, in 

 which numerous islands were scattered about in a manner somewhat 

 similar to what is observed in the coralline zones of the Pacific Ocean. 

 It has been further urged, in support of this view, that in the 

 opinion of many conchologists, notably, Mr. Gwyn Jeffreys, all, or 

 nearly all, the Mollusca of the Cretaceous sea represent comparatively 

 shallow-water forms, there being a total absence of such shells as 

 could with positiveness be considered as pointing to a deep-sea 

 habitat. Numerous objections, however, interpose themselves to 

 the views so ingeniously framed by Mr. Wallace. In the first place, 

 it would be purely gratuitous to assume that a comparatively shal- 

 low coralline sea extended into a continental area whose expanse 

 equalled the tract covered by the chalk deposits of Eurasia ; sec- 

 ondly, had such a sea existed we should naturally expect to find, 

 as has been urged by Mr. Starkie Gardner, the remnants of ancient 

 coral reefs, such as are at the present day being formed in the 



