140 H. FOSTER BAIN 
to determine, if possible, the exact conditions under which the 
country rock was laid down, so as to know what the original 
content of the rock in mineral is likely to have been. Since the 
conditions under which deposits accumulate are reflected in the 
character of the material accumulated, lithology is of primary 
importance at this stage of the investigation. In the second 
place it is important to determine the changes which the rock 
has undergone since its original formation. This at once leads 
into the general study of metamorphism, and again the expres- 
sion of the results is best accomplished by lithologic mapping. 
Since, however, different materials under the same or different 
processes of metamorphism may produce rocks which in their 
final stages are lithologically identical, a map to express the full 
facts must be constructed so that by means of various colors, 
patterns, symbols, or prefixes in the legend, the lithologically 
similar but historically different rocks may be readily distin- 
guished. The structure of the region must be thoroughly 
understood before any general plan of development can be 
formulated. All these facts can and should be represented upon 
any geological map intended for the use of a mining popula- 
tion. 
In general the mining geologist is called upon to do two 
things: (1) to report upon some mine or tract of presumably 
mineralized land in order to determine whether the showing 
warrants the undertaking of development work; (2) to direct 
the further development of mines already partially opened up. 
In reporting upon properties he is oftenest sent to regions which 
are very little developed and which generally are not geologi- 
cally mapped. In such a region he must rely in the first 
instance upon such reconnoissance maps as may be available, 
but in the main upon his own efforts. In the nature of the case 
the first detailed mapping in mineral districts must in a majority 
of instances be the result of individual initiative. The maps 
made in this way are more detailed than official maps can be 
expected to be, but they are also entirely unrelated, and each covers 
a very limited territory. So far, then, as the primary develop- 
