332 WHITMAN CROSS 
Samuel Allport ; Clarence King, 1878; M. E. Wadsworth, 1884. 
J. D. Dana, 1878; Karl A. Lossen; H. Rosenbusch, 1887; H. Rosenbusch, 1896. 
Justus Roth, 1883; E. Kalkowsky, 1886; J. J. Harris Teall, 1886-1888. 
Franz Schrockenstein, 1886, 1897; H. O. Lang, 1891; F. Loewinson-Lessing, 
1890, 1897; A. Osann, 1900, 1901. 
Ferdinand Zirkel, 1893-1894; H. Rosenbusch, 1898; Johannes Walther, 1897. 
SUMMARY. 
Part T. 
INTRODUCTION. 
THE systematic and descriptive science of rocks — petrog- 
raphy —is a product of the nineteenth century. One hundred 
years ago the distinction had not yet been drawn between the 
rock and the geological formation or terrane, and many dense 
rocks were still included among minerals and described as such. 
With the discrimination between stratigraphic units and rocks. 
proper the science of petrography became outlined, but did not 
at first receive aname. For some time after the proper scope of 
the science was clearly defined its development was rapid, and 
in certain later periods there have been notable advances, due to 
the invention of some new method of research or to the stimu- 
lation afforded by discoveries in some closely allied branch of 
science. 
For the last thirty years or more research in all directions. 
has added greatly to our knowledge of rocks, through the 
adaptation of the microscope to their study, improved methods. 
of chemical analysis, and a vast store of accurate field observa- 
tions of occurrence and relationships. But the systematic part. 
of the science has not kept even pace, and does not adequately 
express the knowledge of the day. This lagging behind on the 
part of the classificatory branch of petrography is most natural, 
and similar conditions have existed in various epochs of the 
past. It is due partly to the necessity for a thorough consider- 
