ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 287 
seems to have been aware of Vogelsang’s prior publication. . The first 
two statements referred only to geological occurrence and to textural 
and mineralogical peculiarities; while Iddings, writing at a time 
when the chemistry of rocks had begun to assume its due prominence 
(largely owing to the earnest labors of the chemists of the United 
States Geological Survey), showed that the relationship is also indi- 
cated by the chemical composition of the various rocks, and is funda- 
mentally dependent on this, and he consequently devotes much space 
to the chemical evidence of consanguinity. 
The doctrine of consanguinty, as it may be termed, has now received 
general acceptance, and it is commonly recognized that the igneous 
areas of the earth’s surface are divisible into districts the rocks of 
which show certain features in common which serve to distinguish 
them from those of other districts. It is also assumed that the posses- 
sion of these common features, especially those dependent on the 
chemical composition, indicates that the rocks of a given district 
have a common genetic origin; that is, are derived from a common 
parent body of magma. The processes by which this differentiation 
and derivation from a common magma are brought about are still 
obscure, and form the subject of much modern investigation and dis- 
cussion, into which we can not enter here. Such areas of related 
rocks are usually called petrographic provinces, though the term 
comagmatic region, which indicates more clearly their derivation 
from a common magma, has recently been introduced.* 
Though petrographic provinces represent one of the prominent 
phases of the distribution of the elements in the earth’s crust, and 
though their existence is, in general, undeniable, yet their characters 
are so complex and made up of so many factors that the characteriza- 
tion in most cases can not be made very precise or the limits very 
sharply drawn. Their study is still almost in its infancy, and the 
accumulation of many more data, especially from the analytical side, 
is needed before definitive studies can be undertaken, and the charac- 
ters of any petrographic province be precisely defined. 
The geological evidence of consanguinity is at times clear. Thus, 
as Iddings? says: 
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, would be enough to justify 
the belief that there was a definite connection between the members of a 
group. 
On the other hand, cases are known where such geological evi- 
dence is lacking or conflicting, or where the relations are so generally 
observed as to be meaningless in this connection. 
“H. S. Washington, Carnegie Publication No. 57, p. 5 (1906). 
Loc. cit., p. 43: 
