4 ANALYSES OF ROCKS AND MINERALS, 1880-1908. 
For example, an analysis of the older type says nothing of titanium 
and phosphorus, while the microscope reveals the presence of sphene 
and apatite. In this case the petrographer has been more thorough 
than the chemist. On the other hand, a full and perfect analysis may 
be given, accompanied by a petrographic description of the most 
general kind, in which only the main mineral constituents of the rock 
are noted. Here the analysis has been incompletely used and the 
petrographic discussion is defective. It is hoped that the publication 
of this material may lead to a clearer recognition of the mutuality 
which should exist between the chemical and the microscopic re- 
searches, and so bring, in the future, both lines of investigation more 
into harmony. Hitherto the chemist and the petrographer have 
worked too much apart and each has too often misunderstood the 
purpose of the other. If the study of the thin section could always 
precede the analysis, the petrographic problems could be stated more 
clearly and the chemical evidence might be rendered much more 
pertinent and satisfactory. 
During the preparation of this bulletin much assistance was ren- 
dered by the petrographers and geologists connected with the Survey, 
especially with reference to analyses hitherto unpublished. In each 
case credit has been given for the data thus added. Twenty-eight 
analyses of rocks from Montana, executed by or under the direction 
of Prof. L. V. Pirsson, of Yale University, and made in connection 
with regular Survey work, are included in the tabulations. With 
this exception all of the analyses given were made in the Survey’ 
laboratories. To those executed in the laboratory at Washington 
“record numbers” are attached, which serve to identify them on the 
record books of the division of chemistry. Of the abbreviations 
used for bibliographic reference only five need explanation, and they 
refer to the official publications of the Survey. ‘Ann.” for Annual 
Report, ‘“Mon.” for Monograph, “Bull.” for Bulletin, ““P. P.” for 
Professional Paper, and “‘ Folio,” are the five 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 petro- 
graphic reference collection of the Survey and are followed by the 
number assigned to the rock in that series. 
THE AVERAGE COMPOSITION OF ROCKS. 
In a paper published some years ago,* on the relative abundance of 
the chemical elements, I computed the average composition of the 
primitive crust of the earth from 880 analyses of eruptive and crystal- 
@ Bull. U.S. Geol. Survey No. 78, 1891, p. 34. 
