CHAP. XV.] OF CONTINENTS AND OCEANS. 329 



off the eastern and southern coasts of North America, the 

 purely calcareous deposits being confined to the areas 

 warmed by the G-ulf Stream ; and the fact is confirmed by 

 the observations of Wallich, Carpenter, and Wyville Thom- 

 son. Now it is probable that the European Cretaceous sea 

 was considerably warmer than the present Atlantic, and con- 

 sequently it is not surprising to find a greater relative pro- 

 portion of calcareous matter in the Chalk. 



Again, some beds of Chalk are very largely composed of 

 minute fragments of Inoceramus shell ; the molluscs which 

 formed these shells must have been very abundant on the 

 floor of the Chalk sea, but they have long been extinct, and 

 no mollusc with a similar shell is found abundantly on 

 modern ocean-beds ; consequently this kind of chalk has a 

 special composition, which is apparent under the micro- 

 scope, and which causes its chemical analysis to differ from 

 that of modern calcareous ooze. 



But in spite of these and some other differences there is 

 sufficient resemblance between the analyses of the calcareous 

 ooze and of certain beds of Chalk to make it probable that 

 the original state of the latter was very similar to the pre- 

 sent state of the former. In the first place, we must re- 

 member that the calcareous ooze still retains its full pro- 

 portion of disseminated silica, whereas this silica has clearly 

 been abstracted from the greater part of the Chalk, and 

 has been concentrated into the flint nodules. If, therefore, 

 the ooze be compared with that part of the Chalk which 

 contains flints, the soluble silica should be deducted and 

 the proportions of the other ingredients re-calculated. 



It should be also observed that in ordinary analyses of 

 the ooze there is a much larger proportion of moisture and 

 organic matter, often entered as " loss on ignition," than in 

 analyses of chalk. If, therefore, this loss and the silica be 

 taken into account, the proportion of carbonite of lime to 

 the remaining ingredients is at once made much larger. 



