clarke] INTRODUCTION. 13 
chilly, partial analyses are common ; but in the igneous group thorough- 
ness is more general. In the early days of the chemical division many 
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. Latterly, 
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 elements, 
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 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 petro- 
graphic 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 researches, 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. 
In a paper published some years ago, 1 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- 
line rocks. Of these analyses only 207 were from the laboratories of the 
survey, while 673 were collected from various other American and for- 
eign sources. A large proportion of them were incomplete, regarded 
from a modern point of view, and yet the results obtained were fairly 
conclusive. The material now available for similar discussion is much 
i Bull. U. S. Geol. Survey No. 78, 1891, p. 34. 
