26 PRINCIPLES OF PALZONTOLOGY. 
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. 11 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 
