THE FOSSILIFEROUS ROCKS. 21 
of lime is soluble in water holding a certain amount of car- 
bonic acid gas in solution ; and it is, therefore, found in larger 
or smaller quantity dissolved in all natural waters, both fresh 
and salt, since these waters are always to some extent charged 
with the above-mentioned solvent gas. A great number of 
aquatic animals, however, together with some aquatic plants, 
are endowed with the power of separating the lime thus held 
in solution in the water, and of reducing it again to its solid 
condition. In this way shell-fish, crustaceans, sea-urchins, 
corals, and an immense number of other animals, are enabled 
to construct their skeletons ; whilst some plants form hard 
structures within their tissues in a precisely similar manner. 
We do meet with some calcareous deposits, such as the 
“stalactites” and “stalagmites” of caves, the ‘“ calcareous 
tufa” and “‘travertine” of some hot springs, and the spongy 
calcareous deposits of so-called “petrifying springs,” which 
are purely chemical in their origin, and owe nothing to the 
operation of living beings. Such deposits are formed simply 
by the precipitation of carbonate of lime from water, in con- 
sequence of the evaporation from the water of the carbonic 
acid gas which formerly held the lime in solution ; but, though 
sometimes forming masses of considerable thickness and of 
geological importance, they do not concern us here. Almost 
all the limestones which occur in the series of the stratified 
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 pre- 
sumption 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 presump- 
tion 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 pulver- 
ulent 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 
