THE 
AMERICAN GEOLOGIST. 
VOL. XXXV. FEBRUARY, 1905. No. 2. 
THE COARSENESS OF IGNEOUS ROCKS AND ITS MEANING. 
Alfred C. Lane, Lansing-, Mich. 
PLATE IV. 
For some time* and in a series of papers I have been 
interested in studying the grain of rocks, its variation in 
coarseness and the inferences that may be drawn therefrom. 
Though I began studying it by noticing the dimensions of 
cross sections of certain crystals in thin sections it has be- 
come evident to me that in many cases microscopic work 
and thin sections are not necessary nor even definite numer- 
ical results (fhough always advisable) in order to draw in- 
ferences of interest. I wish therefore in this article to give 
briefly without either the mathematical foundations or the 
exact mathematical limitations,' some of the results of my 
studies in such shape as to attract the attention and interest 
of geologists generally to the value of inferences from the 
coarseness of grain. I should only premise, that ii any one 
finds that the inferences here drawn do not agree with facts 
known to him of which he is absolutely sure, it is quite as 
likely that I have left out, in restating the facts without 
mathematics, some essential limiting factor as that the gen- 
eral theory is seriously defective. There is however one 
presupposition I make, which is not strictly true, variations 
from which may cause great variations in the results. That 
is, that the whole mass of the igneous rock when it was 
♦Part 1, volume vi, Report.s Geological Survey of Michigan; Bull 
Geol. Soc. Am. pp. 369-406; viii, pp. 403-407; Annual Reports, for 1903 and 
1904, Mich. Geol. Surv. Atn. Jour, of Science, Vol. xiv, Nov., 1902, p. 393. 
The plate herewith given is prepared for the Annual Report for 1904. 
