468 THE DEPTHS OF THE SEA. [cHap. x, 
to microscopical examination, many observers were 
struck with the great similarity between its compo- 
sition 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 
