GLACIAL FEATURES 277 



meridian I was fortunate enough to have a topographer the 

 heights of whose stations enabled me to correct the readings of 

 my aneroid barometer whenever these were taken off the line of 

 traverse. Furthermore, the greater height of the passes — 15,000 

 to 17,600 feet — brought me more frequently above the snowline 

 than had been the case in Bolivia and Chile. More detailed ob- 

 servations were made, therefore, not only upon the elevation of 

 the snowline from range to range, but also upon the degree of 

 canting of the snowline on a given range. Studies were also made 

 on the effect of the outline of the valleys upon the extent of the 

 glaciers, the influence on the position of the snowline of mass ele- 

 vation, precipitation, and cloudiness. 



Snow first appears at 14,500 feet (4,320 m.) on the eastern 

 flanks of the Cordillera Vilcapampa, in 13° south latitude. East 

 of this group of ridges and peaks as far as the extreme eastern 

 border of the mountain belt, fifty miles distant, the elevations 

 decrease rapidly to 10,000 feet and lower, with snow remaining 

 on exceptionally high peaks from a few hours to a few months. 

 In the winter season snow falls now and then as low as 11,500 feet, 

 as in the valley below Vilcabamba pueblo in early September, 

 1911, though it vanishes like mist with the appearance of the sun 

 or the warm up-valley winds from the forest. Storms gather 

 daily about the mountain summits and replenish the perpetual 

 snow above 15,000 feet. In the first pass above Puquiura we en- 

 countered heavy snow banks on the northeastern side a hundred 

 feet below the pass (14,500 feet), but on the southwestern or lee- 

 ward side it is five hundred feet lower. This distribution is ex- 

 plained by the lesser insolation on the southwestern side, the im- 

 mediate drifting of the clouds from the windward to the leeward 

 slopes, and to the mutual intensification of cause and effect by 

 topographic changes such as the extension of collecting basins and 

 the steeping of the slopes overlooking them with a correspond- 

 ing increase in the duration of shade. 



It is well known that with increase of elevation and there- 

 fore of the rarity of the air there is less absorption of the sun's 

 radiant energy, and a corresponding increase in the degree of in- 



