16 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
topographic basis for such a map, so great progress lias now been 
made in the preparation of the topographic map that geological map- 
ping is in an advanced stage, and what was formerly called "general 
geology" is now mainly comprised under the term "areal geology." 
Under the folio form, in which the separate sheets of the Geological 
Atlas of the United States are published, there appears, together with 
the topographic and areal maps of the given fraction of a degree which 
it represents, a so-called economic map. The topographic map rep- 
resents the physical relief, or shape of the surface; the areal t map 
indicates by color conventions the area occupied on that surface by 
the different varieties of rocks which constitute the surface, while on 
the economic mail the different rock varieties arc so indicated that 
emphasis is given to those which carry minerals of economic value. 
Thus, the areal geologic work affords results of economic value, and 
is, moreover, an indispensable basis upon which all economic studies 
must be founded. 
ECONOMIC GEOLOGY. 
What was formerly called "mining geology 11 is now designated 
"economic geology," and in late years the scope of this work has so 
greatly increased that it has been found advisable to have it con- 
ducted under two general heads, with a geologist in charge of each, 
namely, that of "metalliferous ore deposits," and that of "lionmetal- 
liferous deposits," those of iron being included in the latter class 
because of their close economic connection with coal deposits. 
The investigation of deposits of metalliferous minerals as at present 
conducted comprises several types of work varying with the condi- 
tions under which it is carried on. These are: First, the investiga- 
tion of important and extensive mining districts, such as Cripple- 
creek, Leadville, etc., which may be called "special district sur- 
veys." These are regions of unusually large concentrations of metal- 
liferous deposits, where within a small area the underground workings 
of a great many large mines have laid open to scientific observation 
relatively large portions of the interior of the earth, and whose indi- 
vidual outputs form a comparatively large fraction of the total 
product of the country. Even in cases where such districts have 
passed their prime from an industrial standpoint, their investigation 
is of the utmost value, since it affords a scientific record of critical 
phenomena which furnish material for the formulating of the general 
laws spoken of above. Hence, this work must be done with the 
highest degree of scientific accuracy and detail, and it generally occu- 
pies the work of at least two field seasons — one by the topographic 
corps in preparing the necessary maps, and one by the geologists in 
making the areal and underground surveys. 
To the general public, and especially to those who own mining 
property in a region, it often seems that the publication of such work 
is unduly delayed, since they are mainly anxious to learn the facts 
