196 ON THE CHEMISTRY OF METAMORPHIC ROCKS. 



that in the earlier periods of the world's history, chemical forces 

 of certain kinds were much more sctive than at the present clay. 

 Thus the decomposition of earthy and alkaline silicates under the 

 combined influences of water and carbonic acid, would be greater 

 when this acid was more abundant in the atmosphere, and when 

 the temperature was probably higher. The larger amounts cf 

 alkaline and earthy carbonates then carried to the sea from the 

 decomposition of these silicates, would furnish a greater amount 

 of calcareous matter to the sediments ; and the chemical effects of 

 vegetation, both on the soil and on the other atmosphere, must 

 have been greater during the Carboniferous period, for example, 

 than at present. In the spontaneous decomposition of feldspars, 

 which may be described as silicates of alumina combined with 

 silicates of potash, soda and lime, these latter bases are removed, 

 together with a portion of silica ; and there remains as the final 

 result of the process, a hydrous silicate of alumina, which consti- 

 tutes kaolin or clay. This change is favoured by mechanical 

 division ; and Daubree has shown that by the prolonged attrition 

 of fragments of granite under water, the softer and readily cleav- 

 able feldspar is in great part reduced to an impalpable powder, 

 while the uncleavable grains of quartz are only rounded, and form 

 a readily subsiding sand ; the water at the same time dissolving 

 from the feldspar a certain portion of silica, and of alkali. It has 

 been repeatedly observed, where potash and soda-feldspars are 

 associated, that the latter is much the more readily decomposed, 

 becoming friable, and finally being reduced to clay, while the or- 

 thoclase is unaltered. The result of combined chemical and 

 mechanical agencies acting upon rocks which contain quartz, 

 wi h orthoclase, and a soda-feldspar such as albite or oligoclase, 

 would thus be a sand, made up chiefly of quartz and potash-feld- 

 spar, and a finely divided and suspended clay, consisting for the 

 most part of kaolin, and of partially decomposed soda-feldspar, 

 mingled with some of the smaller particles of orthoclase and of 

 quartz. With this sediment will also be included the oxide of iron, 

 and the earthy carbonates set free by the sub-aerial decomposition 

 of silicates like pyroxene and the anorthic feldspars, or formed 

 by the action of the carbonate of soda derived from the latter upon 

 the lime salts and magnesia salts of sea-water. The debris of horn- 

 blende and pyroxene will also be found in this finer sediment. 

 This process is evidently the one which must go on in the wearing 

 away of rocks by aqueous agency, and explains the fact that 



