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16 The Microscope in the Examination of Rocks. (January, 
- accomplished by Sorby, Zirkel and Rosenbush may be attributed 
to the introduction of thin sections of rocks for microscopical 
analysis. These sections are ground thin enough to allow the use 
of transmitted light, and although but a small slice of a rock be 
examined, it reveals their composing minerals and their structure 
and also their accessory aggregates. 
The structure and means of cementing of rocks is clearly repre- 
sented in the various sections made in various directions. 
The base of a rock is by aid of the polariscope readily de- 
cyphered, whether it be crystalline or amorphous. The base of 
porphyry is composed of minute particles of feldspar and 
quartz. Basalt was found to contain sometimes enclosures of a 
glassy character, which in many cases are so large that they 
assume the aspect of a base through which the crystalline part is 
scattered, and rocks which were always considered as amorphous, 
were shown by aid of thin sections to be ina state of crystalline 
formation. 
One of the most interesting features of lithology is the chapter 
treating of the cause and result of metamorphic changes in rock. 
A section of an altered rock presents in itself the whole story of 
a process which for a long series of years must have been work- 
ing to produce a chemical and physical alteration in those solid 
bodies. We learn by the study of the thin section with the 
microscope, which of the composing minerals was at first dis- 
turbed and changed, and how the progress of change in the 
molecules was gradually spread through the whole mass. The 
well-known rock, serpentine, may illustrate this. A section presents 
_ outlines of crystals which are on the borders serpentine, but which 
in their centre enclose a clear and unaltered nucleus of chrysolite, 
the remainder of the chrysolite crystal, the form of which is pre- 
served in serpentine. Further, basalt carries chrysolite as one of 
its most common accessory minerals. Nearly all these chryso- 
lites are in a state of metamorphism, their outlines showing 
bands of serpentine, similar in structure to the serpentine occur- 
ring in large masses and the origin of which has been found to 
be ina compound rock changing by ‘the chemical and physical 
alteration to a homogeneous one. 
_A careful microscopical study of rocks and minerals of a coun- 
try enables us also to trace the original rocks which furnished 
those immense layers of drift clay, which when prepared for the 
