THE ORIGIN OF IGNEOUS ROCKS. 
143 
in mineral composition, as they do in chemical composition, 
and their connection with the other rocks of the series in 
which they belong is much more intimate than their sup¬ 
posed connection when placed in an artificial group in ac¬ 
cordance with present methods of classification. 
GEOLOGICAL EVIDENCE OF CONSANGUINITY. 
The geological occurrence of igneous rocks indicates their 
consanguinity, and was the first means by which it was 
recognized. The constant recurrence of particular series of 
rocks, often with a. certain order of eruption in different 
localities, and the frequent occurrence of such series at 
neighboring centers of volcanic activity, sometimes with a 
repetition of the whole or a part of the series, would be 
enough to justify the belief that there was a definite con¬ 
nection between the members of a group; but in regions 
like those of the denuded volcanoes of Electric peak and 
Crandall basin the ph} r sical connection between the varities 
of rock is made evident. The breccias and lava streams, 
with intersecting systems of dikes and intrusive sheets 
joined to a complex core, combine the diverse modifications 
of the whole group of rocks into a geological unit. 
It is to be remarked that the term “ group of rocks ” is 
necessarily somewhat indefinite, since it may be applied to 
all of the rocks derived from one center of eruption, or 
to all those of a number of centers in one region of country 
which can be shown to have been connected and contem¬ 
poraneous. 
There will be found, most probably, a gradual transition 
from a region of one character of igneous rocks to that of 
another, so that no line can be drawn between them. Such 
a transition is indicated in several places, from the rocks of 
the Great Basin to those of the belt east of the Rocky moun¬ 
tains. It is hoped that more will be learned of this eastern 
belt of alkali rocks, and that the geographical distribution 
of the various groups of volcanic rocks all over the world 
