INTRODUCTION. 3 
doubtless be best were petrographers agreed on it; but their differ- 
ences are many, and the chemist will do well to avoid them. The 
geographic method, moreover, has some advantages of its own. It 
facilitates the study of areas, it simplifies the bibliographic refer- 
ences, and it brings together, in great measure, the work of each 
petrographer for whom analyses have been made. Thus, most olf 
Diller’s work has been in California, most of Cross’s in Colorado, and 
most of Iddings’s in the Yellowstone National Park, and in each case 
the analyses are massed, and their discussion is practically uniform 
in character. As regards nomenclature, each rock has received the 
designation given it by its describer and no liberties have been taken. 
This plan may cause some lack of uniformity; but no other pro- 
cedure seemed to be practicable. Whenever it was possible, how- 
ever, 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 published by the Survey in 1903.2 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 thor- 
oughness is more general. In the early days of the chemical divisior 
many analyses were made along the older lines, just as they are stil 
made in many laboratories to-day—that is, only the main con- 
stituents, those having direct petrographic significance, were deter. 
mined. In such analyses the minor ingredients, like titanium, phos. 
phorus, barium, strontium, chlorine, etc., were ignored; and, althougt 
the results were satisfactory in some respects, they left much to be 
desired. Latterly greater completeness has been sought for, the work 
done has been much fuller, and the data obtained can be discussec 
with much higher approaches to accuracy. The old form of ‘‘com- 
plete analysis” is to be discouraged. It leads too often to erroneous 
conclusions, and only the best modern methods of work and of state. 
ment should be tolerated. The fuller analyses, moreover, have 
brought some interesting points to light; titanium now appears to be 
one of the more abundant elements, and barium and strontium are 
found to be almost universally diffused in igneous rocks in quit 
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 noi 
always hold, and the departures from it are in two opposite directions 
@ Professional Paper No. 14. Abbreviations in Professional Paper No. 28. 
