28 THE CHEMISTRY OF METAMORPHIC ROCKS. [III. 



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 further deprived of alkali, metamor- 

 phism 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, staurolite and iron-alumina garnet 

 take their place. The matrix of all of these minerals is gen- 

 erally a quartzose mica-schist. The last term in this exhaustive 

 process appears to be represented by the disthene and pyrophyl- 

 lite 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, 

 besides a variable proportion of silicates or carbonates of lime, 

 magnesia, and oxide of iron. The result of the processes already 

 described will produce a gradual diminution in the amount 

 of alkali, which is chiefly soda. So long as this predominates, 

 the metamorphism of these sediments will^ive rise to feldspars 

 like oligoclase, labradorite, or scapolite (a dimetric feldspar); 

 but in sediments 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, 

 dichroite and chloritoid are formed when magnesia and iron 

 replace lime. In all of these cases the excess of the silicates 

 of earthy protoxides over the silicate of alumina is represented 

 in the altered strata by hornblende, pyroxene, olivine, and 

 similar species ; which give rise, by their admixture with the 

 double aluminous silicates, to dioiite, diabase, euphotide, i-klo- 

 gite, and similar compound rocks. 



In eastern North America, the crystalline strata, so far as 

 yet studied, may be conveniently classed in !iv<- groups, corre- 

 sponding to as many different geological series, four of which 

 will be considered in the present paper. 



