i8o THE ROMAN COMAGMATIC REGION. 
has pointed out, these are all undoubtedly complex in most cases, if not in all, 
especially when differentiation over large areas is involved, and the difficulties in 
the way of solution of the problems are certainly great. But the discovery of such 
similarities as have been shown above, and the undoubted occurrence of similar- 
ities of a different sort obtaining between similar comagmatic regions of widely 
distinct characters from the two here discussed, indicate the truth of Pirsson's con- 
clusion that "the distribution and occurrence of igneous rocks are not due to mere 
chance," and that it is unreasonable "to deny that they are governed like other 
things in nature by definite laws and processes. " 
A much larger body of data than is yet available must be collected before we 
are in a position to ascertain the many facts of differentiation and to understand 
its laws. This is true even of the smaller and less complex rock-masses, such as 
dikes and laccoliths, where the chemical and physical factors involved are com- 
paratively simple. It is a fortiori still more true of such large areas and masses as 
those of comagmatic regions. In these we are confronted with such difficulties 
in the way of complexities of distribution and structure; complexity, variety, and 
mass of magmas; the possibility and, indeed, the probability of superposed and 
diverse processes and conditions; considerations of space and time; the certainty 
of some known physical factors, as viscosity and convection currents, and the 
probability of some unknown ones entering into the problem; that we must for the 
present be content to observe and collect facts, leaving their collation and the final 
generalizations from them for a future period. 
