196 
IDDINGS. 
rocks in any given region, which have been erupted in vari¬ 
ous geological epochs since Archaean or pre-Cambrian times, 
belong to chemically similar groups, and have been derived 
essentially from the same magmas; hence the establishment 
of the chemical character of these magmas must have ante¬ 
dated Cambrian time. 
It has been suggested elsewhere that these differences of 
composition may be connected with differences in the dynam¬ 
ical processes which have led to their eruption—a speculation 
which calls for a more thorough knowledge of the geological 
history of the various regions in which such groups of rocks 
occur. 
The study of the petrology of igneous rocks, as already 
pointed out, leads to conclusions regarding the physical con¬ 
dition of the interior of the earth which are important, be¬ 
cause they are tests of the conclusions derived from purely 
physical considerations. They may be summed up as fol¬ 
lows : 
The phenomena of volcanic eruption and those of the 
crystallization of igneous rocks, as well as the phenomena of 
differentiation, indicate that the magmas from which igneous 
rocks were derived were molten fluids when these processes 
took place. 
But the occurrence at various geological periods of similar 
series of differentiations which were coeval with the course 
of dynamical events during each period of volcanic activity 
indicates that the general or undifferentiated magma was 
solid during long lapses of time—that is, it existed in a state 
of potential liquidity. 
The coordination of all of the characteristics of igneous 
rocks, together with a knowledge of the physical laws which 
have produced them, will furnish the most material evidence 
we may expect to obtain of the true nature of the interior of 
the earth. 
The incompleteness of the evidence which has been pre¬ 
sented in this paper is one of its most obvious features, and 
emphasizes the importance of more thorough investigation 
