ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 301 
a marked effect on the crystallization of the mass, the formation of 
pegmatites, and so forth. It may be argued that such mineralizing 
and pneumatolytic elements are universally, and presumably quite 
uniformly, distributed among the rock magmas, and that their pres- 
ence in the highly siliceous and sodie rocks is due to the greater vis- 
cosity of these when molten, which would hinder the escape of gaseous 
constituents, while the basic magmas are more fluid at low tempera- 
tures and would hence allow such gases to escape before or during 
consolidation. On the other hand, it may be urged that the unde- 
niable distribution among magmas of distinctly different general 
chemical characters, of elements to which no such mineralizing or 
pneumatolytic réle can be reasonably assigned, as barium, beryllium, 
zirconium, titanium, manganese, nickel, chromium, and platinum, 
would lead to the inference that the apparent distribution of the 
gaseous and “ mineralizing ” elements in igneous rocks is real and not 
dependent on physical causes. The subject is highly complex, and 
our knowledge of the fundamental facts and of the physico-chemical 
laws involved is as yet inadequate for solution of the problem, further 
discussion of which would be outside the scope of this paper. 
V. PRACTICAL CONSIDERATIONS. 
When the facts of the relations of the occurrence of the rarer 
elements to the chemical characters of igneous magmas are considered 
it is evident that their distribution over the earth’s surface must be 
largely determined by that of the petrographic provinces. In other 
words, in any given petrographic province those rarer elements and 
minerals containing them would be most apt to occur abundantly 
which show a correlative tendency to association with the character- 
istic major constituents of the province. Thus, zirconium-bearing 
minerals and those of the “ rare earths ” should be most abundant in 
provinces whose magmas are highly sodic and where such rocks as 
nephelite-syenite and phonolite are common; while chromium, nickel, 
and platinum would not be expected in these, but would rather 
be likely to occur in provinces where such rocks as gabbros and 
peridotites are the prevailing ones. 
This idea has been recognized by Spurr? in his proposed term of 
“metallographic provinces,” which is based largely on ore associa- 
tions, and which he apples more especially to those metals of most 
economic importance, such as gold, silver, copper, lead, and zine. 
The probable very close connection between “ petrographic” and 
“ metallographic ” provinces is pointed out by him, but the two classes 
seem to be regarded by him as distinct, at least to a certain extent. 
4Trans., 33, 336 (1903) ; Professional Paper No. 42, U. 8S. Geological Survey, 
p. 276 (1905) ; and No. 55, p, 128 (1906). ; 
