20 THE CHEMISTRY OF METAMORPHIC ROCKS. [III. 



may perhaps to a certain extent become to the geologist what 

 organic remains are in the unaltered rocks, a guide to the 

 geological age and succession. 



It was while engaged in the investigation of metamorphic 

 rocks of various ages in North America, that this problem sug- 

 gested itself; and I have endeavored from chemical considera- 

 tions, conjoined with multiplied observations, to attempt its 

 solution. In the Quarterly Journal of the Geological Society 

 of London for 1859 (Essay II. of the present volume) will be 

 found the germs of the ideas on this subject, which I shall 

 endeavor to explain in the present paper. It cannot be doubted 

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

 of certain kinds were much more active than at the present 

 day. 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 

 (page 2). The larger amounts of alkaline and earthy carbon- 

 ates 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 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 constitutes 

 kaolin or clay. This change is favored by mechanical division ; 

 and Daubree has shown that by the prolonged attrition of frag- 

 ments of granite under water, the softer and readily cleavuMo 

 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, 



