By means of the microscope, the true
nature and mode of formation of chalk can be determined with
the greatest ease. In the case of the harder varieties, the
examination can be conducted by means of slices ground down to
a thinness sufficient to render them transparent; but in the
softer kinds the rock must be disintegrated under water, and the
_debris_ 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 important part in the process of lime-making than perhaps
any other of the larger inhabitants of the ocean.
[Illustration: Fig. 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.
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