emmons. J INVESTIGATION OF METALLIFEROUS ORES. 17 
that directly aid in the development of their own property; but from 
the point of view of those engaged in the work, and who are respon- 
sible for a correct determination of the facts of nature, it is more 
essential that these facts, upon which future generalizations must be 
based, should be determined with the greatest possible accuracy than 
that the public demand for prompt publication should be yielded to. 
The second class of economic work may be called economic work 
incidental to areal work. In regions that are under areal survey it 
often happens that there are considerable mining developments, though 
the important mines are not gathered together into one small area or 
district, but occur at points so widely separated throughout the region 
that it would be inadvisable to survey the whole area with the amount 
of detail that is given to the work in special districts. In such cases, 
after the areal surveys have been completed and published, economic 
geologists are detailed to study the various mine developments of the 
area with a view to the determination of facts of structure and gene- 
sis of the ore deposits examined rather than of their immediate com- 
mercial value. Such are the reports on the Telluride quadrangle by 
Mr. Purington, and on the Silverton quadrangle by Mr. Ransome. 
A third class of economic work is the reconnaissance examina- 
tions, in which it is not intended to make a complete or exhaustive 
examination of a mine or district, but such a characterization as may, 
in a comparatively short time, bring out its most striking and evi- 
dent features, both structural and genetic. Here again the primary 
object, from the point of view of the Survey, is the gathering of facts 
bearing upon the broader questions of structure and origin. As to 
the practical bearing of such work in determining the probable value 
in depth of individual deposits in a region which is still in the pros- 
pect stage, mining men are apt to have somewhat exaggerated ideas. 
While a geologist who has had wide field experience in studying 
mining districts should be able to draw more valuable conclusions as 
to the future prospects of a region as a whole than the prospector or 
miner, as to an individual deposit, until it can be studied underground 
over a considerable extent, both vertically and longitudinal^, he can 
not, as a rule, obtain such scientific data as will enable him to give 
an authoritative estimate of its probable value. 
It has hitherto not been the policy of the Survey to publish this 
reconnaissance work in all cases. It has been considered that, inas- 
much as the very fact of publication of a report by the Survey gives 
to its statement a measure of official indorsement by the Government, 
and as courts of law have accorded to such publications an authority 
as evidence in mining cases equal to that of a text-book on geology 
or mining, incomplete material or opinions that are confessedly 
liable to be changed or modified by more complete studies should not 
be accorded the dignity of a Survey publication. Certain parts of 
this work, foi instance the rapid examinations of mining districts by 
Bull. 213—03 2 
