50 
AMERICAN GEOLOGY. 
feldspathic rocks. Graystone contains seventy-five per cent of 
feldspar—the rest is pyroxene. Euphotide is composed of 
labradorite and sausurite, according to Rose. 
The feldspathic group of rocks is quite extensive, including 
sienite, greenstone, basalt, porphyry, trap, dial]age rock, doler- 
ite, graystone, sausurite in euphotide. 
Mica, although associated with feldspar in granite, can not 
be regarded as one of its constant companions: it rarely accom¬ 
panies it except in the granites of the older periods. The 
following analyses express the compositions of two kinds of 
mica: 
Mica. 
Lepidoiile. 
Silex, 
46-3G 
49-86 
Alumina, 
36-80 
33-61 
Lithia, 
3-60 
Potash, 
9-22 
418 
Fluoric acid and water, 
1-81 
3-45 
Oxide of manganese, 
1-45 
do. iron, 
4-53 
4-18 
The composition of mica is very variable; the iron amounts 
to fourteen per cent. The presence of mica in a rock promotes 
its disintegration mechanically. Its structure favors the 
entrance of water into the mass of which it is a constituent 
part. But this is not all; it contains potash, and hence, like 
feldspar, it is subject to decomposition. Granites, therefore, 
composed of large folia of mica, and large crystals of feldspar, 
are less stable and more subject to crumble than rocks com¬ 
posed of carbonate of lime, or which consist of a simple mineral 
as hornblende; or if the granite is composed of fine particles, 
it is more permanent than the coarser varieties. The Quincy 
and Maine granites are of this description. 
Mica is very rarely a constituent of the pyroplastic rocks. 
The oldest lavas of Etna contain it, and a mass of metamorphic 
magnesian limestone of the Onondaga salt group occurs near 
Syracuse, and is associated with serpentine, another product of 
metamorphic action. 
Feldspar seems to be associated with certain vitreous mine- 
