18 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
heads of divisions for the purpose of determining whether or not they 
should be the subject of official examination in the immediate future, 
are primarily not intended for publication. Again, visits are often 
made to a number of different districts for the purpose of determining 
certain isolated and special facts that bear upon some important 
generalizations under consideration, and their immediate publication 
might defeat the end for which they were made. 
Facts of general interest, with regard either to special mines or to 
mining districts, or generalizations from facts gathered in studies of 
a great many mines or districts, but which are of a more or less tenta- 
tive nature, have been published from time to time by members of 
the Survey, under authorization of the Director, in some scientific 
publication, such as the Transactions of the American Institute of 
Mining Engineers, thus reaching directly and without delay the class 
of persons most immediately interested in them, namely, the mining 
engineers. 
The practical working of this system was well illustrated during 
the Washington meeting of the American Institute of Mining Engin- 
eers in 1900. At this meeting papers were read by different members 
of the Survey, as the result of their independent observations during, 
a series of years, on the following subject s : 
Some Principles Controlling Ore Deposition, by C. R. Van Hise. 
Secondary Enrichment of Ore Deposits, by S. F. Emmons. 
Enrichment of Gold and Silver Veins, by W. H. Weed. 
Motasomatic Processes in Fissure Veins, by W. Lindgren. 
These papers presented theoretical views upon the processes 
involved m the formation of ore deposits which the respective authorsi 
had been gradually arriving at during their Survey work. In most 
cases they represented rather preliminary statements, made for the 
purpose of stimulating discussion and investigation among mining 
engineers, than final and completed results, such as would be expected; 
from an official publication. Yet this prompt publication has been of 
the utmost practical importance to mining industry, for the first thr 
papers give a scientific means of answering a question of the most 
vital interest to the investor in mines, to which, in spite of all that has 
been written on it, only vague and contradictory answers had hitherto 
been presented — the question, namely, whether veins (or ore deposits) 
become richer or poorer with increasing depth. The answer was not 
categorical, for such answers are seldom possible in so complicated a 
science as geology, but it explained the manner of formation of the 
very rich bonanzas which have made certain mines famous and why 
they are succeeded by leaner ores in depth. 
The stimulation of discussion and investigation, which was the gen- 
eral purpose of such papers, has been so fully accomplished in thi&j 
case that they have been followed by a series of important contribu-i 
tions from the most eminent authorities on the study of ore deposits 
