GRANITE. 3 
a common origin, and whether they have any genetic connection with 
the sheared structure which is found in both cases, is not certain; but 
the absence of such porphyritic development in the massive granite is 
significant. 
Microscopic structure—'he essential constituent minerals of the granite, as 
seen under the microscope, are quartz, feldspar (mostly microcline), biotite, 
and muscovite, and it thus belongs to the rocks classed by Rosenbusch 
simply as granite, or ‘‘granite proper.” No hornblende or augite was found. 
As accessory constituents there are magnetite, apatite, and zircon. Most 
of the minerals are usually comparatively fresh; but there is a slight kao- 
linization of the feldspars and an alteration of the biotite, resulting in the 
abstraction of iron, which is first concentrated along the cleavages of the 
mineral and then carried out into the rest of the rock. The feldspar usually 
shows microcline structure, the partial development of which can sometimes 
be seen in a crystal which otherwise has the characteristics of orthoclase, 
indicating that the structure has been induced by pressure, as suggested by 
Rosenbusch. 
The gneissic varieties of the granite have under the microscope a struc- 
ture in general like that of the massive rock. In the fine-grained variety 
mentioned above there are some slight but interesting metamorphic changes. 
The mica in this rock is both biotite and muscovite, occurring intergrown, 
and comparatively fresh. The usual accessory minerals, apatite and zircon, 
are present chiefly as inclusions in the quartz, feldspar, and mica, and tour- 
maline is found in small quantities. The occurrence of this mineral is 
interesting, since it was not found in the massive granite, and is therefore 
probably a product of metamorphism. Specular iron, red hematite, and 
earthy limonite, all evidently secondary, are present, and give the red color 
to the rock, although the structure is granitic, yet the effect of the strain 
which has operated to produce the gneissic arrangement, and probably the 
lenticular aggregates of mica, is plainly visible. The cleavage of the feld- 
spar is strongly developed, sometimes resultmg in slight faulting. Within 
the quartz grains the effect has been to produce fine, straight fractures, which 
are intermittent instead of continuous, and the different sets characteristic- 
ally form isolated crosses. These fractures are most numerous in the 
center of the grain, whence they diminish in frequency toward the edges. 
(See fig. 10, p. 229.) 
