ratified rocks are, primarily at any
rate, of _organic_ origin, and have been, directly or indirectly,
produced by the action of certain lime-making animals or plants,
or both combined. The presumption as to all the calcareous rocks,
which cannot be clearly shown to have been otherwise produced,
is that they are thus organically formed; and in many cases this
presumption can be readily reduced to a certainty. There are
many varieties of the calcareous rocks, but the following are
those which are of the greatest importance:--

_Chalk_ is a calcareous rock of a generally soft and pulverulent
texture, and with an earthy fracture. It varies in its purity,
being sometimes almost wholly composed of carbonate of lime,
and at other times more or less intermixed with foreign matter.
Though usually soft and readily reducible to powder, chalk is
occasionally, as in the north of Ireland, tolerably hard and
compact; but it never assumes the crystalline aspect and stony
density of limestone, except it be in immediate contact with
some mass of igneous rock. 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 sat