GEOGRAPHY OF NORTH AMERICA. 217 



the expedition of 1877 entered the field early in May, with a 

 force aggregating forty men, divided into three sections. Six 

 main and four minor parties traversed and gathered map- 

 material in portions of Colorado, New Mexico, Texas, Utah, 

 Wyoming, Idaho, Montana, Nevada, California, and Oregon. 



A special feature of the expedition has been the elabora- 

 tion of the over- and under-ground survey of the Washoe 

 mining region, containing the well-known Comstock lode. 

 The contour of the entire district has been completed, and 

 will be delineated on a scale of one inch to five hundred feet; 

 while the profiles at distances of one hundred feet, showing 

 the position of vein-matter, ore-bearing bodies, and adjacent 

 country rock, have been well advanced. The position and 

 extent of the drifts have been determined at most levels, and 

 an extended longitudinal section of the entire vein is nearly 

 completed. In this labor the mining superintendents and 

 local engineers have greatly assisted Mr. J. A. Church, M.E., 

 in charge of the work, by contributions from their store of 

 detailed maps, which show the underground openings of all 

 the prominent mines. This special survey, when terminated, 

 will furnish a complete analysis of all branches of silver-min- 

 ing as conducted at this peculiarly interesting mining centre, 

 typical of its kind, and will supplement the admirable work 

 in this same district made a few years ago by Mr. Clarence 

 Kino-. 



Data were also gathered for constructing a detailed topo- 

 graphical map of the Lake Tahoe region, in the Sierra Neva- 

 da, on a scale of an inch to a mile. The other field parties 

 were engaged in surveys necessary for obtaining material 

 for a topographical map, on a scale of one inch to eight miles, 

 of the entire western mountain interior. 



The area covered by the examinations of the geologist, Mr. 

 A. K. Conkling, extended in the Sierra Nevada from San An- 

 dreas and Placerville on the west, to the Como Mountains 

 on the east. This region was found to be composed of igne- 

 ous and metamorphic rocks, to the almost complete exclu- 

 sion of sedimentary rocks. The most common forms are 

 diorite, basalt, hornblende, porphyry, feldspathic porphyry, 

 and volcanic breccia. Many Tertiary Unionida?, and also 

 bird-tracks, were found at the quarry near Carson. Signs 

 of glacial phenomena were noticed in many localities, be- 

 lt 



