THE FOSSILIFEROUS ROCKS. 27 



In the same way, a limestone may be shown to have been an 

 actual coral-reef, by the fact that we find in it great masses of 

 coral, growing in their natural position, 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 com- 

 posed of limestone-rock ("coral-rock"), or of a loose and 

 ("coral-sand"), which is organic in the sense that it is com- 

 posed of lime formed by living beings, but which, in truth, is 

 composed 01 fragments of the skeletons of these living beings, 

 mechanically transported and heaped together by the sea. To 

 take another example nearer home, we may find great accumu- 

 lations 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 mol- 

 luscs, 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 uncommonly 

 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 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 sur- 

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

 and grind these down till they are transparent. When thus ex- 

 amined, 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, 



