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 



