202 



dered on the east by the high bluffs of Bristol Head ; and, on the west, 

 by a ridge and bluffs reaching seven to eight hundred feet above the 

 valley. At the lower end a narrow gate- way leads out to the Rio 

 Grande ; and, at the upper end, a beautiful lake occupies the highest 

 part. Just above this, Clear Creek cuts through the ridge on the west 

 side, and flows out through Antelope Park. The whole mass of this 

 basin has, undoubtedly, fallen in ; and, at one time, Antelope Park must 

 have jutted up against the side of the mountain. We made the ascent of 

 the peak from the sink, but there is a much easier way up the east slope. 

 This was our last station. Passing down the Eio Grande we arrived at 

 J3e] Norte October 10, and thence passing across the San Luis valley 

 and taking the cars at Pueblo, on October 19 we reached Denver, our 

 point of beginning. 



METHODS USED i:^ DETERMHSTIIS'G THE ELEYATIOX OF 

 POINTS IN THE DISTRICT. 



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

 mercurial barometer. Where a standard barometer, whose elevation is 

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

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

 quite impossible to establish a base barometer in the vicinity of the re- 

 gion surveyed, without great expense. All the readings had to be re- 

 ferred to distant stations. Readings 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 

 Fairplay, whose elevation is 9,964,5 feet. The first of these is one hun- 

 dred and fifty miles distant in a straight line from the central part of 

 the San Juan country, while the second is one hundred and twenty-five 

 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 a point several days apart, but finding that 

 the resulting heights, as calculated by reference to those distant bases, 

 did not agree well enough, it was resolved to collect together all the 

 data T)ossible from the field-notes, and see if a fair trigonometric con- 

 nection between the mountain peaks could not be established. The 

 result was, under the circumstances, highly satisfactory. It must be 

 remembered, however, that these observations were not taken with the 

 object in view of making such a system of trigonometric levels. More- 

 over, the instrument used only read to minutes of arc. Supposing an 

 error of a minute in a reading, which is not at all uncommon, the resulting 

 error in the difference of level of two peaks from a single observation 

 will be 15.3 feet for a distance of ten miles and 23 feet for a distance of fif- 

 teen miles. If, as is sometimes the case, the error be more than one 

 minute, the error in the elevation will be still greater. Another large 

 and uncertain element in the problem is refraction, which in the high 

 mountains is so changeable as to add much to the uncertainty of the 

 results. In many cases the observations were taken during storms, and 

 often the peaks were sighted through breaks in the clouds, making the 

 refraction still more uncertain. 



In obtaining the following results, the plan adopted was this : from each 

 station angles of elevation or depression were taken to the surrounding 

 peaks and especially to previous stations. Had the fore sights and back 

 sights between the several stations been simultaneous the error of the re- 

 fraction correction would have been very nearly neutralized, but these two 

 sets of observations were never taken at the same time, and in only one 



