XII EEPORT UNITED STATES GEOLOGICAL SURVEY. 



stations. Between 80 and 90 of these were marked permanently by 

 stone monuments for future reference. 



As heretofore, attention has been paid to the economic value of thd 

 land surveyed. Map notes and sketches indicate the extent of land 

 suitably situated for irrigation, and as all streams of any magnitude are 

 gauged, and their slopes measured, the data for estimates of the amount 

 of arable land are at hand. Areas of pasture and timber lands are also 

 noted. 



During the whole season, of four months' duration, the weather was 

 exceptionally favorable for the prosecution of the work ; scarcely a daj 

 was lost from bad weather by any of the parties. 



The total area surveyed during the season, and to be mapped during 

 the winter, was about 29,000 square miles, a very considerable addition 

 to our knowledge of the Western country. 



In succeeding years the work is to be extended toward the north, 

 east, and west. 



GEOLOGY. 



The geological field-work of the survey for 1877 was assigned to Dr. 

 F. M. Endlich, Prof. Orestes St. John, and Dr. A. C. Peale, in the 

 Sweetwater, Teton, and Green Eiver districts, respectively. Many in- 

 teresting geological facts were observed, which are given in detail in this 

 report. 



The Sweetwater district comprised a well-diversified country. The 

 eastern iDortion of the Green Eiver Basin was found to be underlaid 

 with Tertiary formations, with isolated volcanic eruptions at several 

 points. The prevailing westerly winds of the region have resulted in 

 the formation of sand dunes wherever the configuration of the country 

 has olfered an obstacle to the progress of the sand that is formed from 

 the readily disintegrating Tertiary sandstones. On the eastern side of 

 the Wind Eiver Mountains a full series of the sedimentary formations 

 was noted, beginning with the Silurian, and numerous stratigraphical 

 phenomena were observed and studied with a view to the determination 

 of the age of the mountain-range. 



Camp Stambaugh, at the south end of the range, is located within 

 the area of the oldest metamorphic rocks of the district. In these rocks 

 gold has been found in varying quantities for the last ten years, and, at 

 one time, the region was the scene of considerable mining excitement. 



The western side of the Wind Eiver Mountains was found to be very 

 interesting on account of the remains of enormous ancient glaciers. 

 Moraines, covering many square miles, and often a thousand feet in thick- 

 ness, extend downward through narrow valleys that now contain rush- 

 ing streams. Striation, grooving, and mirror-like polish of rock in situ, 

 denote the course taken by the moving ice-fields that have left these 

 marks of their former existence. From all indications, the cessation of 

 glacial activity must have occurred within a comparatively recent time. 



