302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
When we deal with such complex bodies as veins and other ore de- 
posits, the matter is complicated by such factors as geological struc- 
ture, the existence of faults, the occurrence of the igneous rock as 
plutonic masses, dikes or effusive flows, climatic conditions,‘ and other 
disturbing features. These may tend either to favor or to retard the 
processes of concentration which result in economically exploitable 
metalliferous deposits. But these, though undoubtedly of the highest 
commercial importance, are subsidiary to the more fundamental facts 
of the distribution of the elements in igneous magmas, and it seems 
reasonable to suppose that a study of these latter features should be 
susceptible of results of great practical importance. 
It seems that, at present, the knowledge gained by exact chemical 
analysis that the granites of a certain region contain minute traces of 
gold or of copper would be of little use in guiding one in the search for 
the location of a gold or a copper mine. The prospector must always 
remain a valuable, indeed an invaluable, member of the mining fra- 
.ternity. We can not enter here into the vast and vexed subject of the 
genesis of ore deposits, but if it were known by future researches that, 
for instance, gold or copper is normally associated with magmas of a 
certain general chemical character, a knowledge of this might con- 
ceivably be of material assistance in a search; not so much by indicat- 
ing the exact position of a favorable location, but, in a more general 
way, by leading the prospector to confine his attention to a given 
region of favorable igneous rocks and to disregard one whose rocks, 
on theoretical grounds, would probably result in loss of time and 
effort. Such a knowledge could be gained not only by the complex 
and laborious methods of accurate and minutely complete chemical 
analysis, but more readily, at least in many conceivable cases, by 
simple petrographical examination and field study of the most abun- 
dant and characteristic rock minerals. 
These considerations, it is true, are scarcely applicable as yet to 
search for such metals as gold, silver, or copper, concerning the mag- 
matic relations of which our knowledge is of the vaguest description. 
But, in view of what has been ascertained by petrographical and 
chemical means of the distribution of other elements, it is not un- 
reasonable to think that we shall eventually obtain well-founded and 
definite knowledge concerning the distribution of these also. Indeed, 
the opinion may be expressed that future petrographers will wonder at 
the fact that, for instance, the presence of such deep-seated and ex- 
tensive deposits of copper as those at Butte and in Shasta County, 
California, was so long unsuspected, and that their discovery came as 
a surprise. 
4H. V. Winchell, Popular Science Monthly, vol. 72, No. 6, pp. 534 to 542 (June, 
1908). 
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