334 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



the shores of a continent, the source and starting- 

 point of the material carried down by the rivers ; 

 (2) it must possess a base so little rigid as to allow [ 

 the gradual subsidence of the terrestrial crust at 

 that point, partly, probably, under the actual weight 

 of the sediments accumulated at the bottom of 

 the geo-synclinal. 



The consequences of this subsidence, often un- 

 interruptedly continued for several successive geo- j 

 logical periods, are not difficult to appreciate. When 

 subsiding, the lower part of this series of layers, i 

 sometimes piled on each other to a thickness of 

 several kilometres, gets nearer to the terrestrial 

 crust, and is subjected to higher and higher tem- 

 peratures. One must even suppose that if the 

 movement of subsidence is sufficiently prolonged, 

 these sedimentary layers come into more or less 

 direct contact with a part of the internal magma, ; 

 imprisoned, in a state of fusion and at temperatures 

 of from 1.500 to 2.000 C, below the solid crust, 

 which constitutes, over a thickness of only a few 

 kilometres, a thin pellicule on the surface of the 

 terrestrial globe. Drawn into such a medium, the 

 sediments are subjected to the action of vapours of 

 high temperature, alcaline, fluoric, boric, etc., all 

 endowed with the most intense chemical activity. 

 These vapours impregnate the sediments through 

 the thousand fissures which penetrate them, and 

 combine with the silicious, argillaceous, or calcareous 

 matter of which they are constituted, so as to trans- 

 form it into various silicates, such as feldspar, mica, 

 amphibolites, and pyroxenes, which are precisely 

 the crystallized mineral components of the granitic 



