CHAPTER XII. 



PETROGRAPHY AND METALLOGRAPHY. 



I. 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 difificulty. 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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