7 .-- Section of Gravesend Chalk, examined by transmitted light and highly magnified.

Besides the entire shells of _Globigerina_, _Rotalia_, and _Textularia_, numerous detached chambers of _Globigerina_ are seen.

(Original.)] As chalk is found in beds of hundreds of feet in thickness, and of great purity, there was long felt much difficulty in satisfactorily accounting for its mode of formation and origin.
By the researches of Carpenter, Wyville Thomson, Huxley, Wallich, and others, it has, however, been shown that there is now forming, in the profound depths of our great oceans, a deposit which is in all essential respects identical with chalk, and which is generally known as the "Atlantic ooze," from its having been first discovered in that sea.

This ooze is found at great depths (5000 to over 15,000 feet) in both the Atlantic and Pacific, covering enormously large areas of the sea-bottom, and it presents itself as a whitish-brown, sticky, impalpable mud, very like greyish chalk when dried.

Chemical examination shows that the ooze is composed almost wholly of carbonate of lime, and microscopical examination proves it to be of organic origin, and to be made up of the remains of living beings.
The principal forms of these belong to the _Foraminifera_, and the commonest of these are the irregularly-chambered shells of _Globigerina_, absolutely indistinguishable from the _Globigerinoe_ which are so largely present in the chalk (fig.