Some rocks are entirely made up of matter that once was alive, or formed part of living organisms. This is the case with some limestones, which consist of microscopic shells, or of larger shells, corals, and similar calcareous organisms, either entire or broken into fragments and cemented together with pasty or crystalline limestone filling their interstices. This may be seen in Fig. 9, which represents a magnified slice of a Silurian limestone. Coal in like manner consists of carbonised vegetable matter, retaining more or less perfectly its organic structure, and sometimes even the external forms of its constituent parts. More frequently, fossils are dispersed more or less sparsely through the substance of beds composed of earthy matter; and they have usually been more or less affected by chemical changes, or by mechanical pressure, or are mineralised by different substances which have either filled their pores by infiltration or have more or less completely replaced their substance. Of course, as a rule, the softer and more putrescible organic matters have perished by decay, and it is only the harder and more resisting parts that remain. Even these have often yielded to the enormous pressure to which they have been subjected, and if at all porous, have been changed by the slow action of percolating water charged with various kinds of mineral matter in solution. [Illustration: FIG. 9.—Section of Trenton limestone, magnified, showing that it is composed of fragments of corals, crinoids, and shells. Montreal.] [Illustration: FIG. 10.—Diagram showing different state of fossilisation of a cell of a tabulate coral (Dawson’s _Dawn of Life_). _a_ Natural condition, wall calcite cell empty. _b_ Wall calcite, cells filled with the same. _c_ Walls calcite, cells filled with silica or a silicate. _d_ Wall silicified, cells filled with calcite. _e_ Wall silicified, cell filled with silica.] It thus happens that many fossils are infiltrated with mineral matter. Wood, for example, may have the cavities of its cells and vessels filled with silica or silicates, with sulphide or carbonate of iron, or with limestone, while the woody walls of the cells may remain either as coaly matter or charcoal. I have often seen the microscopic cells of fossil wood not only filled in this way, but presenting under a high power successive coats of deposit, like the banded structure of an agate.