THE UNITED STATES GEOLOGICAL SURVEY. 489 



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 min- 

 eral 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 geo- 

 logic 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 for- 

 mations. 



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 geo- 

 graphic distribution has been carefully mapped in the old dis- 

 tricts and also in new districts, where prospecting had not yet 

 shown them to exist. 



Second. The discovery of the laws which control the occur- 

 rence 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 



