26 PRINCIPLES OF PALAEONTOLOGY. 



tion, and exhibiting plain proofs that they were simply quietly 

 buried by the calcareous sediment as they grew ; but other 

 limestones may contain only numerous rolled and water-worn 

 fragments of corals. This is precisely paralleled by what we 

 can observe in our existing coral-reefs. Parts of the modern 

 coral-islands and coral-reefs are really made up of corals, dead 

 or alive, which actually grew on the spot where we now find 

 them ; but other parts are composed of a limestone-rock 

 ("coral-rock"), or of a loose sand ("coral-sand"), which is 

 organic in the sense that it is composed of lime formed by 

 living beings, but which, in truth, is composed of fragments 

 of the skeletons of these living beings, mechanically trans- 

 ported and heaped together by the sea. To take another 

 example nearer home, we may find great accumulations of 

 calcareous matter formed in place, by the growth of shell-fish, 

 such as oysters or mussels ; but we can also find equally great 

 accumulations on many of our shores in the form of " shell- 

 sand," which is equally composed of the shells of molluscs, but 

 which is formed by the trituration of these shells by the 

 mechanical power of the sea-waves. We thus see that though 

 all these limestones are primarily organic, they not uncom- 

 monly become "mechanically-formed" rocks in a secondary 

 sense, the materials of which they are composed being formed 

 by living beings, but having been mechanically transported to 

 the place where we now find them. 



Many limestones, as we have seen, are composed of large 

 and conspicuous organic remains, such as strike the eye at 

 once. Many others, however, which at first sight appear com- 

 pact, 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 lime- 

 stones, 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 innumerable entire or fragmentary 

 fossils, cemented together by a granular or crystalline matrix 

 of carbonate of lime (figs, n and 12). 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 crystallisa- 

 tion has been carried to a great extent, the original organic 

 nature of the rock may be greatly or completely obscured 



