CHAPTER XII. 
PETROGRAPHY AND METALLOGRAPHY. 
1. The Study of Rock Sections——The determination 
of minerals can be more satisfactorily carried out by the 
microscopic study of thin sections than by any other 
method, and the microscope occupies almost as important 
a position in the geological laboratory as in that of the 
biologist. Frequently optical characteristics are the 
only ones which can be relied on for the identification of 
a given mineral, and almost always they are more easily 
made out than the chemical composition. The presence 
and character of minute impurities are detected with 
facility under the microscope, and much information may 
be obtained with regard to the physical conditions which 
attended the genesis of rock formations. The study of 
the minute structure of fossils forms a subordinate but 
attractive branch of the general subject. 
For examination with the microscope, it is necessary 
to prepare sections of rocks so thin as to be transparent; 
and this is a task of some little difficulty. When the 
necessary apparatus is at hand, slices are first cut with a 
lathe, using for a saw a thin disc of iron, copper, or 
sheet tin charged along the edge with diamond-dust 
or fine emery. It is, however, generally possible to 
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