If) THE GRANITES OF MAINE. 
o-lass, has been effected in the laboratory, and the chief mineral con- 
stituents of granite have been artificially crystallized at high tem- 
perature in the presence of water vapor under high pressure, but the 
conditions requisite for the production of a granitic rock from its 
chemical constituents have not yet been successfully imitated. 
Some granite shows locally a certain alignment of its mica plates 
and feldspars, due to the flow of the mass while it was in a plastic 
s t a te— a structure which was probably controlled by the pressure and 
form of the bordering rock. This "flow structure" should not be 
confounded with the schistosity which is due to later pressure and 
which also involves mineral changes and is usually regional rather 
than local in extent. 
The great differences in the grade of texture in granites — the min- 
eral particles ranging from an average diameter of one-fiftieth inch 
(0.50 mm.) and even 0.0069 inch (0.175 mm.) to over half an inch — 
is attributed to differences in the rate of cooling. The portions at 
the margin of the mass, which cooled rather quickly, crystallized in 
very small crystals, while the central portions, which cooled more 
gradually, became coarsely crystalline. 
M I NERALOGICAL COMPOSITION. 
Feldspar is the mosl conspicuous and generally the most abundant 
mineral in granite. By its color or colors it usually determines to a 
large extent the general color of the rock; and by the light which it 
reflects it causes also its brilliancy. It is easily distinguished from 
the other constituents by its smooth cleavage surfaces and milky, 
bluish white, or opalescent, or reddish, brownish, or greenish color. 
Granite usually contains two kinds of feldspar, the mosi abundanl of 
which is generally potash feldspar, a silicate of alumina and potash. 
This occurs in one of two crystal forms, orthoclase or microcline, or 
in both, which, however, can be distinguished only by means of the 
microscope. The other feldspar (plagioclase), containing both soda 
and lime, although it may be of the same color as the potash feldspar, 
can often be distinguished from it by the very fine parallel lines on 
its surface. Usually it differs greatly in color from the first. The 
potash feldspar may be reddish or brownish; the plagioclase may be 
white or greenish. Under the microscope the soda-lime feldspar can 
be readily distinguished from the potash feldspar by its behavior in 
polarized light, which brings out its crystalline 'structure and indi- 
cates its particular variety and approximate chemical composition. 
A granitic rock that contains the two feldspars in equal proportions 
is distinguished by a special technical name. 
Quartz (silica), the next most abundant constituent, is readily 
known by its glassy luster, uneven fracture, and brittleness. It may 
