METAMORPHIC ROCKS. 
577 
mica-schist to overlie gneiss. But although such an order 
may prevail throughout limited districts, it is by no means 
universal. To this subject, however, I shall again revert, in 
Chapter XXXV., where the chronological relations of the 
metamorphic rocks are pointed out. 
Principal Metamorphic Eocks. —The following may be enu¬ 
merated as the principal members of the metamorphic 
class:—gneiss, mica-schist, hornblende-schist, clay-slate, chlo¬ 
rite-schist, hypogene or metamorphic limestone, and certain 
kinds of quartz-rock or quartzite. 
Gneiss ,—The first of these, gneiss, may be called stratified 
—or by those who object to that term, foliated—granite, be¬ 
ing formed of the same materials as granite, namely, feld¬ 
spar, quartz, and mica. In the specimen here figured, the 
white layers consist almost exclusively of granular feldspar, 
with here and there a speck of mica and grain of quartz. 
The dark layers are composed of gray quartz and black mica, 
with occasionally a grain of feldspar intermixed. The rock 
Fig. 622 . 
Fragment of gneiss, natural size; section made at right angles to the 
planes of foliation. 
splits most easily in the plane of these darker layers, and the 
surface thus exposed is almost entirely covered with shining 
spangles of mica. The accompanying quartz, however, great¬ 
ly predominates in quantity, but the most ready cleavage is 
determined by the abundance of mica in certain parts of the 
dark layer. Instead of consisting of these thin lamina3, gneiss 
is sometimes simply divided into thick beds, in which the 
mica has only a slight degree of parallelism to the planes of 
stratification. 
Hand specimens may often be obtained from such gneiss 
w^hich are undistinguishable from granite, afibrding an argu¬ 
ment to which we shall allude in the concluding part of this 
chapter, in favor of those who regard all granite and syenite 
not as igneous rocks, but as aqueous formations so altered as 
to have lost all signs of their original stratified arrangement. 
Gneiss in geology is commonly used to designate not merely 
25 
