128 
IDDINGS. 
CONSxVNGUINITY OF IGNEOUS ROCKS. 
The repeated association of different kinds of volcanic 
rocks in various parts of the world and in different volcanic 
districts led Scrope, Darwin, Dana, and others to the belief 
that they were derived from a common source by some pro¬ 
cess of separation. 
On the other hand, the great variability of the rocks 
among themselves in particular regions led others to em¬ 
phasize the points of difference, and to believe that the 
differences must have existed always, or have been the result 
of the mingling of widely different materials. 
The first idea was based on broad geological observations; 
the second was inspired chiefly by chemical and physical 
considerations. Each idea involved the other to a greater 
or less extent, and approached nearer to completeness in 
proportion to the extent to which all of these conditions 
were taken into account. 
At the time of the early speculations regarding the re¬ 
lationship and origin*of igneous rocks little was known of 
the actual mineral composition and crystalline structure of 
rocks beyond what could be gathered from the crystals 
large enough to study with a pocket lens, which, in the case 
of a great proportion of them, is but a part of the whole 
rock. Even the chemical composition of the rocks was im¬ 
perfectly known, owing to the incompleteness and insuffi¬ 
ciency of the chemical methods then in use. 
With the introduction and development of microscopical 
investigation the mineralogical character and the crystalline 
structure of igneous rocks have become better known. The 
first natural result of such detailed study has been to mag¬ 
nify the mineralogical differences of rocks, and to construct 
a complex classification based often on very slight differences, 
which, however, are more apparent as the rocks studied are 
fewer in number and are from different regions. 
In studying all of the igneous rocks of any one region, 
the investigator is at first impressed with the differences in 
