122 
BULLETIN OF THE LABORATORIES 
The impetus given years 2 igo hy Zirkle, in his ‘‘Microscopic Pe- 
trography,” has slowly become apparent. Hawes, in his work upon 
New Hampshire lithology, contributed substantial material to the same 
science, while the more recent reports of the Wisconsin geological 
survey afford evidence that the work is going on. Several of the 
State geological surveys are now in the midst of investigations in this 
direction the results of which may be looked for with great interest. 
The United States surveys are not behind in cultivating the promising 
field. 
Great as is the promise of research in this direction, we are equally 
interested to observe that the introduction of the new method of study 
of rocks is to a degree revolutionizing the study of geology. The 
same methods which have so greatly augmented the disciplinary value 
of biology by connecting histological and laboratory practice with its 
study, are introduced into the courses in geology and the student is 
taught to see th7mtgh^ as well as to look at, rocks and minerals. 
The study of a rock or mineral involves, first, the investigation of 
the origin, age, and relations of the rock, which invoke respectively 
the sciences of geotechnical, historical, and stratigraphical geology; 
second, the study of the rock itself, which may be carried on by means 
of chemical and physical tests. Under the latter head come crystal- 
lography and physical mineralogy. The chemical examination in- 
volves the application of heat, as in blow-pipe analysis, or of chemical 
reagents in the wet way. 
The physical examination of minerals may be conducted micro- 
scopically or macroscopically, depending upon the employment or non- 
employment of aids to ordinary vision. Ordinary physical mineralo- 
gy is occupied with such of the optical or other characters of minerals 
as oan be made out with the unassisted eye. 
In order to prepare a mineral or rock for microscopic examination 
it must, in most cases, be reduced to a transparent condition in order 
that it may be studied by transmitted light. The facts which can be 
obtained by the microscopic examination of opaque masses in reflected 
light, are few and unimportant: 
A rock may be reduced to a powder and mounted in a transparent 
medium and many of its elements detected by microscopic examina- 
tion of the angles and surfaces of the fragments. In this 'way parti- 
cles too small for measurement by the ordinary goniometer may be de- 
termined, Such of the resulting particles as are transparent may 
