work is the particular province of the section of petrology. Rocks may be ground down to sections so thin that they become transparent, and then, with a particular form of microscope, the minerals composing them may be determined.

On the return of the geologist from the field such microscopic sections as are necessary are made and are studied in consultation with the chief of the section of petrology. Analyses of rocks or ore are made in the chemical laboratory, and the fossils collected are submitted to the paleontologists for determination and interpretation. The field notes are plotted, sections are drawn, and the report is written.

Accompanying each report is a geologic map on which is shown, by appropriate colors and symbols, the distribution of the various formations. Geologic time-the time during which the rocks were madeis divided into several large parts called periods. Smaller time divisions are called epochs, and still smaller ones stages. The age of a rock is expressed by naming the time interval in which it was formed. The sedimentary rocks are grouped together into systems. The principal divisions of a system are called series. Any aggregate of formations less than a series is called a group. Where a part of a formation is specially designated it is known as a member. In the work of the Geological Survey the following systems and series are recognized:

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. Pl. II shows a portion of the Telluride, Colorado, sheet. In the foreground is the map, and above it is a structure section. 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 trustworthy 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 complexity and in which fossils are too rare to be of much service is the preCambrian. In the rocks of this age are the great copper and iron mines of the Lake Superior region, the lead deposits of the Cœur d'Alene, many of the copper deposits of Arizona, and the gold-silver veins of the West. A special section of the Survey 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 necessarily 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 structure 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 determined only as a result of the comprehensive study of many mines and mining camps. This work is beyond the reach of individuals, but is