468 THE DEPTHS OF THE SEA. [chap. x. 



to microscopical examination, many observers were 

 struck with the great similarity between its composi- 

 tion and structure and that of the ancient chalk. I 

 have already described the general character and the 

 mode of origin of the great calcareous deposit which 

 seems to occupy the greater part of the bed of the 

 Atlantic. If we take a piece of the ordinary soft 

 white chalk of the south of England, wash it down 

 with a brush in water, and place a drop of the 

 milky product on the slide of a microscope, we find 

 that it consists, like the Atlantic ooze, of a large pro- 

 portion of fine amorphous particles of lime, with here 

 and there a portion of a Globigerina shell, and more 

 rarely one of these shells entire, and a considerable 

 proportion — in some examples coming up to nearly 

 one-tenth of the whole — of ' coccoliths,' which are 

 indistinguishable from those of the ooze. Altogether 

 two slides — one of washed down white chalk, and 

 the other of Atlantic ooze— resemble one another so 

 clearly, that it is not always easy for even an accom- 

 plished microscopist to distinguish them. The nature 

 of chalk can also be well shown, as has been done by 

 Ehrenberg and Sorby, by cutting it into thin dia- 

 phanous slices, when the mode of aggregation of the 

 different materials can be readily demonstrated. 



But while successive observers have brought out 

 more and more clearly those resemblances, — suffi- 

 ciently striking to place it beyond a doubt that the 

 chalk of the cretaceous period and the chalk-mud of 

 the modern Atlantic are substantially the same, — a 

 more careful investigation shows that there are very 

 important differences between them. The white chalk 

 is very homogeneous, more so perhaps than any other 



