102 
AMERICAN GEOLOGY. 
In structure it approaches granite in many districts in the United 
States. In composition the mica may he wholly or in part re¬ 
placed by hornblende, and a part of the feldspar of the rock 
crystallized in large particles. We have then the granitoid 
gneiss, hornblendic and porphyritic gneiss. If mica abounds in 
it, it is either said to be schistose or micaceous gneiss; or if 
quartz, quartzose gneiss. These terms are employed to express 
its structure or composition in any observed locality. Gneiss 
passes into mica slate by the loss of its feldspar. 
The foregoing considerations seem to establish the doctrine 
that gneiss, mica slate, hornblende and talcose slate are really 
contemporaneous formations. They are so blended in all the 
districts of our country, that it is extremely difficult, if not im¬ 
possible, to define their boundaries. Even in the talcose slate 
of the gold region, gneiss frequently appears in wide areas. 
I shall not attempt, therefore, to give the boundaries or extent 
of either of the foregoing rocks, being content with pointing 
out the regions they occupy in common. 
§ 70. Mica slate. This rock consists of mica and quartz. 
The laminae are thinner than those of gneiss, and it has more 
mica in its composition. The feldspar, when it occurs in this 
rock, is in the form of seams or segregation, intermixed with 
a coarser mica than that which composes the body of the rock. 
The quartz is gray, the mica usually black, and the particles 
of both are fine. 
Numerous varieties might be pointed out, which are due to 
variable proportions of quartz and mica, or to the thickness of 
the laminae, or to the intermixture of other minerals, as horn¬ 
blende or talc. These varieties are not so important as to 
require detailed descriptions. It is sufficient that the student 
should know that mica slate passes lithologically into other 
rocks without changing materially its structure. 
§ 71. Talcose slate. It is composed of talc and quartz. Its 
laminae are thinner than those of mica slate. Its color is gray, 
and its luster is more silken than mica slate. Its laminae are 
undulating, curled, or crisped. Its texture is fine, and its feel 
