730 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



mountain has its features and individual forms. It also requires men of 

 great physical endurance to carry on such work, owing to the many 

 difficulties and often dangers that are met with in ascending so many 

 rough and high mountains, without any previous knowledge of the 

 country, and the traveling is often found difficult through these un- 

 known regions, where there is neither track nor trail to guide them in 

 their selection of a route, and being compelled by the nature of the 

 work to reach certain points. 



DETERMINATIONS OF ALTITUDES. 



The altitudes have been determined with mercurial barometers. Each 

 party is generally supplied with two barometers, and with extra tubes 

 and fixtures with which to repair any breakage that may occur. Base 

 barometers have been established at various points over the Territory, 

 always as near the district in which the work was being carried 

 on as practicable, which was often at a greater distance than was de- 

 sirable, the x)arties working often far beyond the borders of civilization. 



The heights of the mountain stations have been constantly checked 

 by a system of vertical angles between all occupied points, thus bind- 

 ing the whole together, so there are but few points depending upon a 

 single reading of the barometer for their heights, except the valleys 

 and such places as could not be thus checked. I give below an extract 

 from the report of Mr. Franklin Rhoda (who was my assistant during 

 the summer of 1874), which will serve to illustrate the method of con- 

 necting the points by vertical angles : — 



" METHODS USED IN DETERMINING THE ELEVATION OF POINTS. 



" All the elevations given in this report depend upon readings of a 

 mercurial barometer. Where a standard baromeler whose elevation is 

 well determined is within a short distaace, this instrument gives a very 

 good determination of elevation. In the past summer, however, it was 

 quite impossible to establish a base barometer ia the vicinity of the 

 region surveyed,, without great expense. All the readiugs had to be re- 

 ferred to distant stations. Eeadings on high peaks were referred to the 

 Signal-Service barometer on Pike's Peak at an elevation of 14,147 feet 

 above the sea, while readings on all points under 12,000 feet were referred 

 to the barometer of the United States Geological Survey at Fair Play, 

 whose elevation is 9,964.5 feet. The first of these is 150 miles distant 

 in a straight line from the central part of the San Juan country, while 

 the second is 125 miles distant. These distances are too great to give 

 accurate results with the barometer. At several points in the region we 

 succeeded in getting two readings at the same point at intervals of sev- 

 eral days, but finding that the resulting heights as calculated by ref- 

 erence to those distant bases did not agree well enough, it was resolved 

 to collect together all the data possible from the field-notes and see if 

 a fair trigonometric connection between the mountain-peaks could not 



