1 8 INTRODUCTION. 



are of large size, varying from the size of a split pea up to 

 that of a florin. Very many limestones, however, are made 

 up of the calcareous cases of much smaller forms of Fora- 

 minifera, which are so minute as hardly to be visible to the 

 naked eye. In other cases, again, we find limestones to be 

 composed so largely of the shells of various of the true 

 Mollusca, that we may regard them as essentially made up 

 of the skeletons of this class of animals. 



At the present day, then, limestone is in process of forma- 

 tion by the agency of various animals, amongst which the 

 Corals, the Foraminifera, and the Mollusca are the most 

 important. The same animals have also been the principal 

 agents in building up the great masses of limestone which 

 we now discover in the crust of the earth ; but in the case 

 of the older calcareous rocks we must add to the above the 

 Crinoids, as having formerly contributed on an immense scale 

 to the formation of limestone. NOT are we only to ascribe 

 an organic origin to such limestones as are composed of 

 fossils large enough to be visible to the unassisted eye. On 

 the contrary, most other limestones which at first sight ap- 

 pear compact, more or less crystalline, and nearly devoid of 

 traces of life, are found, when properly examined, to be also 

 composed of the remains of various organisms. All the 

 commoner limestones, in fact, from the Lower Silurian 

 period onwards, can be easily proved to be thus organic 

 rocks, if we investigate weathered or polished surfaces with 

 a lens, or, still better, if we cut thin slices of the rock and 

 grind these down till they are transparent. When thus 

 examined, the rock is usually found to be composed of in- 

 numerable entire or fragmentary fossils, cemented together 

 by a granular or crystalline matrix of carbonate of lime 

 (tigs. 4 and 5). When the matrix is granular, the rock is 

 precisely similar to chalk, except that it is harder and less 

 earthy in texture, whilst the fossils are only occasionally 

 referable to the Foraminifera. In other cases, the matrix is 

 more or less crystalline, and when this crystallisation has 

 been carried to a great extent, the original' organic nature of 

 the rock may be greatly or completely obscured thereby. 

 Thus, in limestones which have been greatly altered or 



