18 ANALYSES OF ROCKS, U. S. GEOLOGICAL SURVEY. [bull. 168. 
of rocks from Montana, executed by or under the direction of Prof. 
L. V. Firsson, of Yale University, and having been made in connection 
with regular Surve}^ work, are included in the tabulations. With this 
exception all of the analyses given were made in the Survey labora- 
tories. To those executed in the laboratory at Washington " record 
numbers" are attached, whrch serve to identify them on the record 
books of the Division of Chemistry. Of the abbreviations used for 
bibliographic reference only three need explanation, and they refer to 
the official publications of the Survey. "Ann." for Annual Report, 
u Mon." for Monograph, and "Bull." for Bulletin are the three in 
question. The others relate to well-known journals, and are familiar 
to all geologists. The letters P. R. C. , following the description of a 
rock, refer to the Petrographic Reference Collection of the Survey, 
and are followed by the number assigned to the rock in that series. 
