14 INTRODUCTION. 



a certain amount of carbonic 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-men- 

 tioned 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 solu- 

 tion 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 pre- 

 sumption 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 pulveru- 

 lent 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, 



