14 ANALYSES OF KOCKS, U. S. GEOLOGICAL SURVEY. [no. 228. 
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 eA r idence might be rendered much more 
pertinent and satisfactory. 
During the preparation of this bulletin much assistance was rendered 
by the petrographers and geologists connected with the Survey, 
especially with reference to anatyses hitherto unpublished.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 having been 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 four need explanation, and they 
refer to the official publications of the Survey. u Ann." for Annual 
Report, " Mon." for Monograph, "Bull." for Bulletin, and "P. P." 
for Professional Paper are the four 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. 
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 anatyses of eruptive and crystal- 
line rocks. Of these anal} T ses only 207 were from the laboratories of 
the Survey, while 673 were collected from various other American and 
foreign sources. A large proportion of them were incomplete, 
regarded from a modern point of view, and }^et the results obtained 
were fairly conclusive. 
In Bulletin No. 148 a similar estimate was given, based upon 680 
complete analyses found in the Survey records, plus some hundreds of 
determinations of silica, lime, and alkalies. Again, in Bulletin No. 168^ 
a third estimate was presented, representing 830 complete anatyses 
and some partial determinations, all made in the Survey laboratories. 
In 1899 Harker 6 published a computation covering 397 analyses of 
a Bull. U. S. Geol. Survey No. 78, 1891, p. 34. 
?>Geol. Mag., 4th decade, vol. 6, p. 220. 
