172 A. Wade—Oomposition of Igneous Rocks. 
skill, and few petrologists find themselves able to undertake such 
work, though the need for it is becoming more and more apparent. 
A valuable alternative, however, remains. Many remarkably accurate 
and useful results have been obtained by the application of the ‘ Linear 
Traversing Method’! of Rosiwal. By this method the percentage 
mineral composition of the rock is obtained. By multiplying each 
quantity by its density and making use of the known chemical 
composition of the minerals we get a fairly accurate statement of the 
quantitative chemical composition of the rock. 
_ I wish, however, to suggest a method which will be found, I hope, 
more simple, and with a little care equally effective. Instead of the 
ordinary eyepiece with single cross wires one should be obtained fitted 
so as to give a square field. This field should be crossed by wires 
dividing each side of the square into ten equal parts, and the whole 
area therefore into 100 equal divisions. In making an examination of 
any rock section the percentage mineral composition of the part of the 
slide under examination is easily found by simply counting the squares 
occupied by the several mineral species present. By taking a number 
of observations in different parts of the same slide and obtaining an 
average a fairly close approximation to the actual chemical composition 
of the rock from which the section is cut can be obtained by a skilful 
observer. This average would, of course, become more closely approxi- 
mate by considering more than one section cut from the same specimen. 
f 
Field of Eyepiece showing the arrangement of crosswires. 
Thus by means of this simple arrangement a student should be 
expected, not merely to identify the mineral constituents present in 
a rock section, and so obtain a general idea of its relationships, but to 
express his conclusions with regard to both mineral constitution and 
chemical composition by means of number. ‘This would constitute 
a real step in advance in the study of petrology as carried on at present 
in this country, and would probably lay the foundations for a more 
satisfactory system of classifying the igneous rocks than the somewhat 
hazy one now in vogue. 
1 See Rosiwal, Verbuch. Wien Geol. Reichs. Anst., vol. xxxii (1898), or 
“Quant. Classif.’”’ above, p. 204. 
