III.] THE CHEMISTRY OF METAMORPHIC ROCKS. 21 



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

 orthoclase is unaltered. The result of combined chemical and 

 mechanical agencies acting upon rocks which contain quartz, 

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

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

 feldspar, and a finely divided and suspended clay, consisting for 

 the most part of kaolin and of partially decomposed soda-feld- 

 spar, 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 feld- 

 spars, or formed by the action of the carbonate of soda derived 

 from the latter upon the lime-salts and the magnesia-salts of 

 sea-water. The debris of hornblende 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 while quartz, or an excess of 

 combined silica, is for the most part wanting in rocks which 

 contain a large proportion of alumina, it is generally abundant 

 in those rocks in which potash-feldspar predominates. 



So long as this decomposition of alkaliferous silicates is sub- 

 aerial, the silica and alkali are both removed in a soluble form. 

 The process is often, however, submarine or subterranean, tak- 

 ing place in buried sediments which are mingled with carbon- 

 ates of lime and magnesia. In such cases the silicate of soda 

 set free reacts either with these earthy carbonates, or with the 

 corresponding chlorides of sea-water, and forms in either event 

 a soluble soda-salt, and insoluble silicates of lime and magnesia 

 which take the place of the removed silicate of soda. The 

 evidence of such a continued reaction between alkaliferous 

 silicates and earthy carbonates is seen in the large amounts of 

 carbonate of soda, with but little silica, which infiltrating 

 waters constantly remove from argillaceous strata ; thus giving 

 rise to alkaline springs and to natron-lakes. In these waters 

 it will be found that soda greatly predominates, sometimes 

 almost to the exclusion of potash. This is due not only to 

 the fact that soda-feldspars are more readily decomposed than 



