14 PRINCIPLES OF PALAEONTOLOGY. 



of this by no means rnre 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 II. 

 THE FOSSILIFEROUS ROCKS. 



Fossils are found in rocks, though not universally or pro- 

 miscuously ; and it is therefore necessary that the palaeonto- 

 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 Igneous Rocks such as 

 the lavas and ashes of volcanoes 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 



