ON THE CHEMISTRY OF METAMORPHIC ROCKS. 203 



meable and more highly silicious class of sediments already 

 noticed, whose chief elements are silica, alumina, and alkalies, 

 the deposits of different ages will be marked chiefly by a pro- 

 gressive diminution in the amount of potash, and the disappearance 

 of the soda which they contain. In the oldest rocks the propor- 

 tion of alkali will be nearly or quite sufficient to form orthoclaso 

 and albite. with the whole of the alumina present; but as the 

 alkali diminishes, a portion of the alumina will crystallize, on the 

 metamo r phism of the sediments, in the form of a potash-mica, 

 such as muscovite or margarodite. While the oxygen ratio be- 

 tween the alumina and the alkali in the feldspars just named is 

 3 : 1, it becomes 6 : 1 in margarodite, and 12 : 1 in muscovite. The 

 appearance of these micas in a rock denotes then a diminution in 

 the amount of alkali, until in some strata the feldspar almost 

 entirely disappears, and the rock becomes a quartzose mica-schist 

 In sediments still farther deprived of alkali, metamorphism gives 

 rise to schists filled with crystals of kyanite, or of andalusite ; 

 which are simple silicates of alumina, into whose composition 

 alkalies do not enter ; or in case the sediment still retains oxide of 

 iron, staurotide and iron-alumina garnet take their place. The 

 matrix of all these minerals is generally a quartzose mica-schist. 

 The last term in this exhaustive process appears to be represented 

 by the disthene and pyrophyllite rocks, which occur in some 

 regions of crystalline schists. 



In the second class of sediments we have alumina in excess, 

 with a small proportion of silica, and a deficiency of alkalies, be- 

 sides a variable proportion of silicates or carbonates of lime, magne- 

 sia, and oxide of iron. The result of the processes already de- 

 scribed will produce a gradual diminution in the amount of alkali, 

 which is chiefly soda. So long as this predominates, the meta- 

 morphism of these sediments will give rise to feldspars like oligo- 

 clase, labradorite, or scapolite (a dimetric feldspar) ; but in sedi- 

 ments where lime replaces a great proportion of the soda, there 

 appears a tendency to the production of denser silicates, like lime- 

 alumina garnet, and epidote, or zoisite, which replace the soda- 

 lime feldspars. Minerals like the chlorites, and chloritoid, are 

 formed when magnesia and iron replace lime. In all these cases 

 the excess of the silicates of earthy protoxides over the silicate of 

 alumina is represented in the altered strata by hornblende, pyrox- 

 ene, ohvine, and similar species ; which give rise by their admix- 

 ture with the double aluminous silicates, to diorite, diabase, 

 euphotide, eklogite, and similar compound rocks. 



