74. MINERALOGY AND LITHOLOGY. 
of their decomposition. It is very probable, however, that by metamor- 
phic and other agencies garnets might be formed, as they so often are, 
on the junction of siliceous rocks and limestones; but it is on account 
of the proximity of the chemical elements which by reaction on one 
another may form garnets, and is not any proof of the igneous condi- 
tion of either mass. 
Garnet is often present as a microscopic constituent of our rocks. It 
seems likely to exist anywhere, save in the basic eruptive rocks. When 
present in microscopic preparations it is recognized by its optical deport- 
ment, since, owing to its single refraction, its sections, in whatever direc- 
tion they are cut, are dark in all positions between crossed Nicols. In 
the schists the garnets are usually crystalline; and then their sections are 
commonly six- or eight-sided, being sections of dodecahedrons, but often 
they are nearly round. In the granites, on the contrary, the garnets 
frequently have no crystalline outline, but have a most irregular and 
eccentric structure, and look like melted bits of glass. Garnet has no 
cleavage, and its sections are usually traversed by rifts, which go in all 
directions. 
Some beautiful specimens of garnets, all mounted and in readiness for 
microscopic study, are found in our granitic veins. These are garnets 
which, having crystallized between plates of mica, are flattened out, and, 
instead of appearing as natural crystals, look like thin flat discs fastened 
in the mica. They form very pretty objects, and are thin enough to be 
studied with the microscope. Lasaulx* mentions one of these garnets 
which was all penetrated by little crystalline needles of tourmaline. 
Other microscopic impurities, such as quartz, magnetite, and epidote, are 
often present in garnets, and sometimes a garnet will have cavities in it 
of crystalline form. 
The garnets that occur in the hornblende schist at Hanover are very 
interesting as a microscopic study. They are well crystallized in perfect 
dodecahedrons, and to all appearance are perfectly pure, many of them 
being quite clear, and of a beautiful wine color; but when a thin section 
is cut through one of these, it is seen to be filled with grains of quartz, 
which in many cases constitutes at least one third of the whole. Fig. 5 
« Elemente der Petrographie, Dr. A. Von Lasaulx, p. 80. 
