12 GEOLOGICAL SURVEY OF THE TERRITORIES. 



9. Shoshone Geyser Basin, and Lake. 



10. Parts of Idaho and Wyoming about the heads of the Snake 



Eiver. (Scale, five miles to one inch.) 

 (Prepared by G. K. Bechler.) 



11. Parts of Montana and Wyoming about the heads of the Yel- 



lowstone, Gallatin, and Madison Elvers. (Scale, four miles 

 to one inch.) Drawn by Henry Gannett from field-notes by 

 A. Burck. 



12. Part of Colorado, based on the United States land-survey. 



1873. Compiled for field use. 



No. 11, containing the complete list of the Ornithology of theli^Torth- 

 west, is passing rapidly through the press. Nos. 14, 18, 19, 20, 22, 23, 

 24, 25, and 26 are either ready for the press or in an advanced state of 

 preparation, and will be issued within a year. 



The map of a portion of Montana and Wyoming Territories, embrac- 

 ing most of the country about the sources of the Madison, Gallatin, and 

 Yellowstone Elvers, in contour lines of 100 feet, and on a scale of four 

 miles to one inch, can hardly be said to be published yet, only a few 

 copies having been issued. The data for this map were collected in the 

 field by Mr. Adolph Burck, assisted by Mr. Henry Gannett. In the 

 office it was completed by Mr. Gannett. The following explanation in 

 regard to the method and purpose of the map will certainly be suffi- 

 cient for all fair-minded men. 



The topographical work was carried on mainly from a meandered 

 line, (meandered with compass and odometer.) Points along the line 

 were located by angles, using the meandered line as a base-line. Much 

 topography, also, was done from mountain-peaks, with gradienter and 

 compass. In addition to this, short bases were measured in several 

 localities with steel-tape, and connected with the topographical work, 

 to serve as checks, and the whole was checked at the camps by sextant 

 observations for latitude and time, the longitude being determined by 

 chronometer, the rate of which was repeatedly checked. 



The location of the contours was controlled by barometric observa- 

 tions, vertical angles with the gradienter, and slope angles with the 

 clinometer. While not pretending to be an accurate contour map, 

 which it would be useless to attempt to make in a hasty reconnaissance, 

 it does assume to express, with considerable accuracy, the form of the 

 country, in its \ ertical as well as in its horizontal features. 



The system of plotting maps with elevation curves is of the greatest 

 importance for practical geological studies of any locality or country 

 that has been surveyed in that way. It is evident that wherever sedi- 

 mentary formations occur the curve system greatly facilitates the work 

 of the geologist. Geological maps, if carefully made, will always rep- 

 resent more or less scalloped curved lines, denoting the boundaries of 

 the different geological formations, and it is therefore apparent that 



