clarke.] INTRODUCTION. 13 
analyses are massed, and their discussion is practically uniform in 
character. As regards nomenclature, each rock has received the desig- 
nation given it by its describer, and no liberties have been taken. 
This plan may cause some lack of uniformity; but no other procedure 
seemed to be practicable. Whenever it was possible, however, I have 
inserted in italics the new names proposed by Cross, Iddings, Pirsson, 
and Washington in their classification of the igneous rocks. In most 
cases, these names were taken from Washington's great compilation of 
analyses, recently published by the Survey/' This addition, it is 
believed, will assist petrographers in their study of the material thus 
brought together. 
It will be noticed by anyone who uses this bulletin that the analyses 
vary as regards completeness. Among the sedimentary rocks, espe- 
cially, partial analyses are common, but in the igneous group thorough- 
ness is more general. In the early da} T s of the chemical division man} 7 
analyses were made along the older lines, just as they are still made in 
many laboratories to-day — that is, only the main constituents, those 
having direct petrographic significance, were determined. In such 
analyses the minor ingredients, like titanium, phosphorus, barium, 
strontium, chlorine, etc., were ignored; and, although the results are 
satisfactory in some respects, they leave much to be desired. Latterty, 
greater completeness has been sought for, the work done has been 
much fuller, and the data obtained can be discussed with much higher 
approaches to accuracy. The old form of "complete analysis" is to 
be discouraged; it leads too often to erroneous conclusions; and only 
the best modern methods of work and of statement should be tolerated. 
The fuller analyses, moreover, have brought some interesting points 
to light; titanium now appears to be one of the more abundant ele- 
ments, and barium and strontium are found to be almost universally 
diffused in igneous rocks in quite perceptible quantities. 
On general principles the analysis of a rock and its petrographic 
description should be two parts of the same investigation, matching 
each other completely. In practice, however, this rule does not 
always hold, and the departures from it are in two opposite directions. 
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 b} T 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- 
a Prof essional Paper No. 14, 1903. Abbreviations in Professional Paper No. 28. 
