198 ON THE CHEMISTRY OF METAMORPHIC ROCKS. 



which latter, decaying on the shore, or in the ooze at the bottom, 

 restore the alkali to the earth. The conditions under which the 

 vegetation of the coal formation grew, and was preserved, being 

 similar to those of peat, the soils became exhausted of potash, and 

 are seen in the fire-clays of that period. 



Another effect of vegetation on sediments is due to the reducing 

 or de-oxidizing agency of the organic matters from its decay. 

 These, as is well known, reduce the peroxide of iron to a soluble 

 protoxide, and remove it from the soil, to be afterwards deposited 

 in the forms of iron ochre and iron ores, which by subsequent 

 alteration become hard, crystalline and insoluble. Thus, through 

 the agency of vegetation, is the iron oxide of the sediments with- 

 drawn from the terrestrial circulation ; and it is evident that the 

 proportion of this element diffused in the more recent sediments 

 must be much less than in those of ancient times. The reducing 

 power of organic matter is farther 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 sub- 

 sequent 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 sediment- 

 ary strata is two-fold : — -first, the decomposition of silicates con- 

 taining these bases, such as anorthic feldspars and pyroxene ; and 

 second, the action of the alkaline carbonates formed by the decom- 

 position of feldspars, upon the chlorids of calcium and magnesium> 

 originally present in sea-water ; which have thus, in the course of 

 ages, been in great part replaced by chlorid of sodium. The clay, 

 or aluminous silicate which has been deprived of its alkali, is thus 

 a measure of the carbonic acid removed from the air, of the 

 carbonates of lime and magnesia precipitated, and of the amount 

 of chlorid 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 con- 

 tain ; and if organic matters intervene, the oxide of iron ; leaving 



