ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 303 
At the present time a knowledge of the distribution of the ele- 
ments is practically applicable not so much to the metals of greatest 
human utility as to certain elements whose economic possibilities 
are only recently beginning to be appreciated as their chemical and 
physical properties and the application of these to commercial and 
economic purposes are becoming better known. Some illustrations 
may be permitted of the practical application of the facts set forth 
in the preceding pages. 
If, for instance, one were in a new country or were engaged in a 
search for minerals containing such elements as zirconium, uranium, 
the rare earths, or beryllium, one would welcome a district of highly 
sodie igneous rocks, where albitic granites, nephelite-syenites, and 
phonolites were abundant; in this the chances of success would be 
most favorable. If the rocks were prevailingly gabbros, diabases, 
or feldspar-basalts one would reasonably assume that such minerals 
could not be expected to occur, at least in such amount as to repay 
exploitation, and they would be neglected, or prospected for plati- 
num or chromium, let us say. Similarly, if the platinum metals were 
found in the sands of a river the watershed of which covered areas of 
gabbros, granites, and limestones, one would naturally turn to and 
explore the first in an attempt to trace the grains to their source, and 
would, with good reason, leave the others alone. 
Instances of this kind could be multiplied, and, indeed, some 
present applications of the general principles are now practiced not 
infrequently, but without any suspicion of the true principles under- 
lying them or realization of their more general applicability. Thus, 
in certain districts the occurrence of topaz or spodumene may be 
recognized as generally indicating the possible or probable presence 
of cassiterite, without appreciation of the more general and funda- 
mental fact that the conjunction of tin, fluorine, and lithium is due 
to the distinctly sodic character of the igneous rocks. 
With increase in our knowledge of the origin of ore deposits, and 
a general agreement as to their ultimate source in igneous rocks 
(whatever be the divergence of views as to the processes of concen- 
tration), the probability of the future importance of such observa- 
tions as have been outlined above, from a practical as well as from 
a theoretical standpoint, is fairly evident. We can not as yet predict 
the probable presence of gold, silver, or copper in economic quanti- 
ties from the petrographical and chemical study of the country rock; 
but the time may come (and our increasing knowledge of igneous 
rocks justifies us in a certain degree of confidence that it will come) 
when such seemingly erudite and impracticable studies will be able 
to guide us in certain regions, as to either the probable absence or 
presence of ore bodies of such metals. 
