130 
IDDINGS. 
distinguishes it from every other district. On the contrary, 
there are many that appear to be identical with one another 
and undoubtedly are the same ; but there occur in various 
parts of the world family groups of rocks which, while iden¬ 
tical with other groups, are different from many surrounding 
them and constitute associations of rock groups. The recur¬ 
rence of these associations establishes a still higher order of 
relationship or the existence of a more remote common 
origin. § 
It is to express the idea that all the igneous rocks of any 
volcanic district have been derived from a common stock— 
that is, from a common magma—that the writer has applied 
the term consanguinity to the relationship. 
MINER ALOGICAL EVIDENCE OF CONSANGUINITY. 
To illustrate what are some of the characteristic qualities 
of the minerals and rocks of groups which are thus allied 
may be mentioned a few of the more easily recognizable 
features. 
They are partly inherent in the minerals and partly con¬ 
sist in their association in the rock, affecting its habit. To 
a very great extent they depend on the chemical constitution 
of the magma of the rock, though it is not always possible 
to point out the connection, on account of the incomplete¬ 
ness of our knowledge of the chemical composition of the 
minerals in every case, and also because there are other con¬ 
ditions besides chemical ones which have influenced crys¬ 
tallization. 
One of the most universal and persistent minerals in vol¬ 
canic rocks is augite. It is one also in which the qualities 
referred to are most noticeable, since they affect its color. In 
a great many groups of rocks, and through a vast extent of 
territory in western America, the color of the augites in thin 
section is pale green, with no recognizable pleochroism or 
difference of color. Rarely it assumes a purplish tone in 
some ophitic basalts; but as the eastern flank of the Rocky 
