JB (DITA OVAL, 
THE preparation of thin sections of rocks, is an art, in which 
tew have suceeded. It appears to require patience in reducing 
the sections at the proper rate, and delicacy of touch in finishing 
them. Those of us who study rocks with a microscope are 
aware how seldom rock sections are prepared of the desired 
thinness for optical investigation. Too often the section has 
been left unfinished, so that the more minute crystals present a 
confused aggregation of indeterminable parts. There is no 
doubt that the difference between good and bad petrographical 
work is due in many cases to differences in the quality of the 
thin sections studied. Sections thin enough to permit the feld- 
-spar and quartz to display only the lowest gray interference 
color, and which reveal the minutest textures in microcrystalline 
or glassy rocks, are essential to satisfactory work 
The petrographers connected with the United States Geo- 
logical Survey have been fortunate in this respect, having been 
furnished with exceedingly well made rock sections, prepared 
for the most part by Hermann Ohm and Frederick C.Ohm. The 
former has left with many of us the memory of a skilled and 
honest worker, who won our respect and friendship; while the 
latter has earned an equally high place in the esteem of those 
who look to him for the preparations by means of which they 
forward their special investigations. It is not too much to say 
that a large part of the pleasure and satisfaction which the writer 
has derived from his petrographical researches is due to the 
excellency of the thin sections made by these workers. 
It is gratifying to learn that Mr. F. C. Ohm has started his 
son in Washington in the business of making rock sections, and 
that he will be able to supervise the finishing of them and can 
guarantee the character of the work. It is to be hoped the son 
will become as expert as his father, and that the undertaking 
will prove successful, since it will be of great benefit to students 
of petrography throughout the country to have the opportunity 
of obtaining thoroughly satisfactory rock sections. J kale 
354 
