ON THE CHEMISTRY OF METAMORPHIC ROCKS. 197 



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 longas 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, taking place 

 in buried sediments, which are mingled with carbonates of lime 

 and magnesia. In such cases the silicate of soda set free, re-acts 

 either with these earthy carbonates, or with the corresponding 

 chloridsof 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 

 bot 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 decom- 

 posed than orthoclase, but to the well-known power of argillaceous 

 sediments to abstract from water the potash salts which it already 

 holds in solution. Thus when a solution of silicate, carbonate, 

 sulphate, or chlorid of potassium is filtered through common 

 earth, the potash is taken up, and replaced by lime, magnesia, or 

 soda, by a double decomposition between the soluble potash salt 

 and the insoluble silicates or carbonates of the latter bases. Soils 

 in like manner remove from infiltrating waters, ammonia, and phos- 

 phoric and silicic acids, the bases which were in combination with 

 these being converted into carbonates. The drainage-water of soils, 

 like that of most mineral springs, contains only carbonates, chlorids, 

 and sulphates of lime, magnesia, and soda: the ammonia, potash, 

 phosphoric and silicic acids being retained by the soil. 



The elements which the earth retains or extracts from waters 

 are precisely those which are removed from it by growing plants. 

 These, by their decomposition under ordinary conditions, yield 

 their mineral matters again to the soil ; but when decay takes 

 place in water, these elements become dissolved, and hence the 

 waters from peat bogs and marshes contain large amounts of 

 potash and silica in solution, which are carried to the sea, there 

 to be separated — the silica by protophy tes, and the potash by algae, 



