14 PRINCIPLES OF PALAZZONTOLOGY. 
of this by no means rare phenomenon, is that the wood must 
have undergone a slow process of decay in water charged with 
silica or flint in solution. As each successive particle of wood 
was removed by decay, its place was taken by a particle of 
flint deposited from the surrounding water, till ultimately the 
entire wood was silicified. The process, therefore, resembles 
what would take place if we were to pull down a house built 
of brick by successive bricks, replacing each brick as removed 
by a piece of stone of precisely the same size and form. ‘The 
result of this would be that the house would retain its primi- 
tive size, shape, and outline, but it would finally have been 
converted from a house of brick into a house of stone. Many 
other fossils besides wood—such as shells, corals, sponges, 
&c.—are often found silicified; and this may be regarded as 
the commonest form of fossilisation by replacement. In other 
cases, however, though the principle of the process is the same, 
the replacing substance may be iron pyrites, oxide of iron, 
sulphur, malachite, magnesite, talc, &c.; but it is rarely that 
the replacement with these minerals is so perfect as to preserve 
the more delicate details of internal structure. 
CHAPTER: if. 
THE FOSSTILIFEROUS ROCKS. 
Fossils are found in rocks, though not universally or pro- 
miscuously ; and it is therefore necessary that the paleonto- 
logist should possess some acquaintance with, at any rate, those 
rocks which yield organic remains, and which are therefore 
said to be “ fossiliferous.” In geological language, all the 
materials which enter into the composition of the solid crust 
of the earth, be their texture what it may—from the most im- 
palpable mud to the hardest granite—are termed “ rocks ;” 
and for our present purpose we may divide these into two great 
groups. In the first division are the Zegweous Rocks—such as 
the lavas and ashes of voleanoes—which are formed within the 
body of the earth itself, and which owe their structure and 
origin to the action of heat. The Igneous Rocks are formed 
primarily below the surface of the earth, which they only reach 
as the result of volcanic action ; they are generally destitute of 
distinct ‘‘ stratification,” or arrangement in successive layers ; 
and they do not contain fossils, except in the comparatively 
