22 PRINCIPLES OF PALEONTOLOGY. 



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 con- 

 struct 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 consequence of the evaporation from the 

 water of the carbonic acid gas which formerly held the lime in 

 solution ; but, though sometimes forming masses of con- 

 siderable 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 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 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 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 



