30 THE UNITED STATES GEOLOGICAL SURVEY. [bull. 227. 
The character and age of each rock mass are shown on the map, and 
by means of structure sections the relations of the different masses 
are indicated. PI. II shows a portion of the Telluride, Colorado, 
sheet. In the foreground is the map, and above it is a structure sec- 
tion. Such a map and section give a fairly complete account of the 
geologic history of the area in a very condensed form. 
From the study of fossils and of the characters of the rocks trust- 
worthy ideas may be obtained as to the climatic conditions of the past. 
It has been learned, for example, that Greenland and the northern 
countries which are now covered by perpetual snow and ice formerly 
had a warm climate, and that at a later time the northern portion of 
the United States, extending down about to the Ohio and Missouri 
rivers, was covered by a great ice sheet similar to that now covering 
Greenland. The records of this ice period are very complete, but also 
very confusing; fossils help but little in making out the history, and 
because of its complexity a special section of the Survey is devoted to 
its study. 
Another portion of the geologic section which is of great complex- 
ity and in which fossils are too rare to be of much service is the pre- 
Cambrian. In the rocks of this age are the great copper and iron 
mines of the Lake Superior region, the lead deposits of the Cceur 
d'Alene, many of the copper deposits of Arizona, and the gold-silver 
veins of the West. A special section of the Surve} 7 is accordingly 
devoted to the study of the stratigraphy of the pre-Cambrian rocks. 
Six monographs have been prepared describing the iron ranges, and one 
the copper-bearing rocks, of the Lake Superior region. The mining 
districts of the West are being studied by this section in connection 
with the two sections devoted to economic geology. 
The economic work of the Survey has from the first been made 
very prominent. In mapping the geologic formations the relation of 
the rocks to the ore bodies is necessaril} 7 studied in considerable detail. 
The distribution, mode of occurrence, and genesis of the ores are also 
investigated, in order that their economic importance may be properly 
estimated and their development carried on with the greatest economy. 
Two sorts of data result from these studies — first, those which throw 
light on the general laws governing the formation of ores and other 
minerals of economic importance; and second, facts of geologic struc- 
ture and occurrence which are immediately useful in the development 
of the particular ore bodies studied. 
The Survey has consistently occupied the position that its greatest 
and most peculiar usefulness lay along the first-mentioned line of 
research. The principles controlling mineral deposition and the laws 
governing the occurrence of ore bodies can be satisfactorily deter- 
mined only as a result of the comprehensive study of many mines and 
mining camps. This work is beyond the reach of individuals, but is 
