22 PRINCIPLES OF PALZONTOLOGY. 
of slices ground down to a thinness sufficient to render them 
transparent ; but in the softer kinds the rock must be disinte- 
grated under water, and the aébris examined microscopically. 
When investigated by either of these methods, chalk is found 
to be a genuine organic rock, being composed of the shells or 
hard parts of innumerable marine animals of different kinds, 
some entire, some fragmentary, cemented together by a matrix 
of very finely granular carbonate of lime. Foremost amongst 
the animal remains which so largely compose chalk are the 
shells of the minute creatures which will be subsequently 
spoken of under the name of Foraminifera (fig. 7), and which, 
in spite of their microscopic 
dimensions, play a more im- 
portant part in the process of 
lime-making than perhaps any 
other of the larger inhabitants 
of the ocean. 
As chalk is found in beds 
of hundreds of feet in thick- 
ness, and of great purity, there 
was long felt much difficulty 
in satisfactorily accounting for 
its mode of formation and ori- 
gin. By the researches of 
Carpenter, Wyville Thomson, 
Fig. 7.—Section of Gravesend Chalk, Huxley, Wallich, and others, 
examined by transmitted light and highly .- 
magnified. Besides the entire shells of it has, however, been shown 
Cubierrina, Retela, ond Textulerie that there is now forming, in 
gerina are seen. (Original.) 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 Globigerine 
which are so largely present in the chalk (fig. 8). Along with 
these occur fragments of the skeletons of other larger creatures, 
