III.] THE CHEMISTRY OF METAMORPHIC EOCKS. 23 



diffused in the more recent sediments must be much less than 

 in those of ancient times. The reducing power of organic mat- 

 ter is further shown in the formation of metallic sulphurets ; 

 the reduction of sulphates having precipitated in this insoluble 

 form the heavy metals, copper, lead and zinc, which, with iron, 

 appear to have been in solution in the waters of early times, 

 but are now by this means also abstracted from the circulation, 

 and accumulated in beds and fahlbands, or by a subsequent 

 process have been redissolved and deposited in veins. All 

 analogies lead us to the conclusion that the primeval condition 

 of the metals, and of sulphur, was, like that of carbon, one of 

 oxidation, and that vegetable life has been the sole medium of 

 their reduction. 



The source of the carbonates of lime and magnesia in sedi- 

 mentary strata is twofold : first, the decomposition of sili- 

 cates containing these bases, such as anorthic feldspars and 

 pyroxene ; and, second, the action of the alkaline carbonates 

 formed by the decomposition of feldspars, upon the chlorides of 

 calcium and magnesium originally present in sea-water ; which 

 have thus, in the course of ages, been in great part replaced by 

 chloride of sodium. The clay, or aluminous silicate which has 

 been deprived of its alkali, is thus at once a measure of the 

 carbonic acid removed from the air, of the carbonates of lime 

 and magnesia precipitated, and of the amount of chloride of 

 sodium added to the waters of the primeval ocean. 



The coarser sediments, in which quartz and orthoclase prevail, 

 are readily permeable to infiltrating waters, which gradually 

 remove from them the soda, lime and magnesia which they 

 contain ; and, if organic matters intervene, the oxide of iron ; 

 leaving at last little more than silica, alumina and potash, 

 the elements of granite, trachyte, gneiss and mica-schist. On 

 the other hand, the finer marls and clays, resisting the penetra- 

 tion of water, will retain all their soda, lime, magnesia, and 

 oxide of iron ; and containing an excess of alumina, with a 

 small amount of silica, will, by their metamorphism, give rise 

 to basic lime-feldspars and soda-feldspars, and to pyroxene and 

 hornblende, the elements of diorites and dolerites. In this 



