Page:Popular Science Monthly Volume 46.djvu/505

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THE UNITED STATES GEOLOGICAL SURVEY.

three parties in the study of the water supply of the United States, especially in the arid and semi-arid central region; one party in examining the clays, etc., in connection with the areal geology of central Texas; one party in making a reconnaissance of the mineral belt of central Idaho; and two parties in the areal mapping and a study of the gold belt of California. Thus twenty-one parties were engaged in work relating to important resources. It is also planned to continue the study of the phosphate deposits of Florida during the winter, and to begin the mapping of the coal deposits of southwest Oregon and western Washington in the spring of 1895. In addition, work on the geology of highways has been started, and the economic chemical work of the survey has been continued. To provide topographic maps for the geologic work thirty-two topographic field parties were engaged in the various sections of the country.

A typical illustration of geologic economic work is that on the iron-ore deposits of the Lake Superior region. The results include the determination of the geologic position and geographic distribution of the iron-bearing formations, and of the laws which control the occurrence of ore bodies within the iron-bearing formations.

First.—The investigations thus far made show the presence of an iron-bearing formation at the summit of the lower Huronian series, another near its base, and a third at the base of the upper Huronian series which was derived largely from the detritus of the iron-bearing formation of the lower Huronian. Their geographic distribution has been carefully mapped in the old districts and also in new districts, where prospecting had not yet shown them to exist.

Second.—The discovery of the laws which control the occurrence of the ore bodies is of equal if not of greater economic importance than the mapping of the iron-bearing formations. They are as follows: 1. The iron ores always rest upon a relatively impervious basement. 2. Large ore bodies are found only when the impervious basements are in the form of pitching troughs. 3. The pitching troughs are particularly likely to bear unusually large ore bodies when the iron-bearing formation has been much shattered by folding.

By the aid of the areal and structural maps which have been and will be prepared, and the application of the above laws, the mining engineer may avoid unnecessary expenditure of money in exploration, and be guided in the development of the mineral resources of the region. Hydrography. The scope of the work of the hydrographic division of the survey is expressed in the statute authorizing it, which reads: "For gauging the streams and determining the