THE 
JOURNAL OF GEOLOGY 
SALT EMD ER-OGLOBE more 
MICROSCOPICAL PETROGRAPHY FROM THE 
QUANTITATIVE VIEWPOINT 
FRED. EUGENE WRIGHT 
Carnegie Institution of Washington 
INTRODUCTION 
The development of petrology has been along the lines of 
normal growth followed by other sciences. From the Stone age 
on, people have had to do with rocks and have gathered, in the 
course of the centuries, a vast amount of information about them, 
their characteristics, their modes of occurrence, and their uses in 
practical life. Different rock types have received different names, 
and, up to the last century, were classified according to their 
general appearance and the purposes they served: Fine-grained 
rocks, like phonolites, the component minerals of which could 
not be distinguished by the unaided eye, were considered homo- 
geneous and grouped with minerals, until chemists were able to 
show by partial analysis that part of the rock was soluble in 
acid and part insoluble. With the advent of the petrographic 
microscope, a new and fascinating world was opened to the stu- 
dent of rocks, and for some decades the interest was centered in 
the qualitative description of rocks, their mineral composition, 
and their textures. It was virgin land for the petrologist to 
explore, and the methods he adopted were the methods of recon- 
naissance, analogous to those employed by the geologist, who 
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