STRATIGRAPHIC CLASSIFICATION 141 
ment of mineral districts is concerned, reconnoissance maps 
are the best that can be rightly expected by the mining geolo- 
gist. For his purpose it is more important that he should have 
such maps of the whole country, and that such facts as are 
represented on them shall be accurate, than that they should 
show great detail. For example, if such a map shows the 
presence of limestone and porphyry, together with a structure 
generally similar to that of the Leadville district, he will feel 
warranted in having prospecting undertaken within the area. 
The exact distribution of the limestone and porphyry areas is a 
very minor consideration, and errors in this particular will not 
greatly injure the usefulness of the map. A mining company 
must always make independent and careful surveys in any event 
in locating and patenting the property. 
Since it is generally true that rocks of the same age are apt 
to bear the same or related minerals, any suggestion as to the 
age of the rocks becomes immensely helpful. If the age is in 
doubt the fact can be indicated by a question mark in the 
legend, but even a guess is helpful in suggesting the thought of 
one who has studied the territory more widely than is possible 
to one working against time to select the best locations ina 
given district. 
There is apparently a growing sentiment against reconnois- 
sance and general maps. This is unfortunate if the interests of 
the miner are to be taken into account. It is true that such 
maps must in the nature of things be not only incomplete but 
inaccurate. Yet they are the maps which in nearly every case 
are used in the development period of a mining camp, and in 
making them the geologist does the greatest service, quantita- 
tively at least, to the mining industry that he is capable of. It 
is impossible to make an accurate and serviceable geologic map 
of a mining district, such as the maps of the Telluride and Butte 
folios of the United States Geological Survey, in advance of a 
certain amount of mining development. Such general maps, how- 
ever, as are found in the Hayden Adas of Colorado can be made 
readily and at comparatively low cost. They are at once avail- 
