ATMOSPHERIC DUST IN THE ARCTIC REGIONS. 153 



ing both visits I studied the atmospheric effect both from the 

 valleys and from the mountain-tops to above fourteen thou- 

 sand feet. The change in the atmospheric aspects produced 

 by man was striking and obvious. The dust of civilization, 

 the smoke from the numerous smelters, locomotives, and 

 household fires, had so polluted the air that it was scarcely 

 half so transparent as before, and had curiously changed the 

 aspect of the landscapes when seen from the heights 



No height yet reached by man is above this dust, but the 

 higher we go the finer it is, producing corresponding effects. 

 The intensely dark blue of the sky, the color often tinged 

 with violet when seen from high peaks, is due to the fineness 

 of the atmospheric dust at those altitudes. The coarser dust 

 lies nearer the surface. Professor Langley has described it as 

 seen from the summit of Mount Whitney, looking eastward 

 over the deserts, and I have studied it from numerous peaks. 

 The aspect varies with the season, with the weather at the 

 time, and with the time of day. For illustration I may cite 

 my experience on Lassen's Peak, in northern California. 

 The first ascent was made when a cyclonic area was passing. 

 An ocean of cloud was below us, driven by the furious wind 

 and tossed into gigantic billows. We were entirely above 

 this. Its surface was the purest white, illuminated by the 

 sun from a cloudless sky, and eighty miles away the majestic 

 carve of Mount Shasta rose a mile into the clear air above 

 this cloud-ocean. 



The next ascent was a few days later, after the storm had 

 entirely passed, and illustrated the other features. This peak 

 commands a wider view than I have seen elsewhere, extend- 

 ing in some directions to two hundred miles. We made the 

 ascent in the night, and watched the marvellous colors of the 

 dawn over the dusty deserts in the east. West and south of 

 us was the great central valley of California, its objects clear 

 in the early morning, but before midday it was filled with 



