286 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



in composition to the present, by the taking away of certain 

 elements and the addition of certain others. This is the 

 theory of metamorphism by pseudomorphic changes, as they 

 are called, and is the one taught by the now reigning school of 

 chemical geologists, of which the learned and laborious Bischof, 

 whose recent death science deplores, may be regarded as the 

 great exponent. The second hypothesis supposes that the 

 elements of these various rocks were originally deposited as, 

 for the most part, chemically formed sediments, or precipitates ; 

 and that the subsequent changes have been simply molecular, 

 or, at most, confined in certain cases to reactions between the 

 mingled elements of the sediments, with the elimination of 

 water and carbonic acid. It is proposed to consider briefly 

 these two opposite theories, which seek to explain the origin 

 of the rocks in question respectively by pseudomorphic changes 

 in pre-existing crystalline plutonic rocks, and by the crystal- 

 lization of aqueous sediments, for the most part chemically 

 formed precipitates. 



Mineral pseudomorphism, that is to say, the assumption by 

 one mineral substance of the crystalline form of another, may 

 arise in several ways. First of these is the filling up of a 

 mould left by the solution or decomposition of an imbedded 

 crystal, a process which sometimes takes place in mineral veins, 

 where the processes of solution and deposition can be freely 

 carried on. Allied to this is the mineralization of organic 

 remains, where carbonate of lime or silica, for example, fills 

 the porjes of wood. When subsequent decay removes the 

 woody tissue, the vacant spaces may, in their turn, be filled by 

 the same or another species.* In the second place we may 

 consider pseudomorphs from alteration, which are the result of 

 a gradual change in the composition of a mineral species. This 

 process is exemplified in the conversion of feldspar into kaolin 

 by the loss of its alkali and a portion of silica, and the fixa- 

 tion of water, or in the change of chalybite into limonite by 

 the loss of carbonic acid and the absorption of water and 

 exygen. 



* Hunt on the Silicification of Fossils, Canadian Naturalist, New Series, 

 1.46. 



