by the rapidly ascending currents of hot air that rise from the overheated pampas.  During the early afternoon this wind reaches a high velocity and swirls the sand along in clouds.  It is now strong enough to move the heavier particles of sand, uphill.  It sweeps the heaviest ones around the base of the dune and deposits them in pointed ridges on either side.  The heavier material remains stationary at night while the lighter particles are rolled downhill, but the whole mass travels slowly uphill again during the gales of the following afternoon.  The result is the beautiful crescent-shaped medano.

About five o’clock our mules, a fine-looking lot—­far superior to any that we had been able to secure near Cuzco—­trotted briskly into the dusty little plaza.  It took some time to adjust the loads, and it was nearly seven o’clock before we started off in the moonlight for the oasis of Vitor.  As we left the plateau and struck the dusty trail winding down into a dark canyon we caught a glimpse of something white shimmering faintly on the horizon far off to the northwest; Coropuna!  Shortly before nine o’clock we reached a little corral, where the mules were unloaded.  For ourselves we found a shed with a clean, stone-paved floor, where we set up our cots, only to be awakened many times during the night by passing caravans anxious to avoid the terrible heat of the desert by day.

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Figure

Mt.  Coropuna from the Northwest
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Where the oases are only a few miles apart one often travels by day, but when crossing the desert is a matter of eight or ten hours’ steady jogging with no places to rest, no water, no shade, the pack animals suffer greatly.  Consequently, most caravans travel, so far as possible, by night.  Our first desert, the pampa of Sihuas, was reported to be narrow, so we preferred to cross it by day and see what was to be seen.  We got up about half-past four and were off before seven.  Then our troubles began.  Either because he lived in Arequipa or because they thought he looked like a good horseman, or for reasons best known to themselves, the Tejadas had given Mr. Hinckley a very spirited saddle-mule.  The first thing I knew, her rider, carrying a heavy camera, a package of plate-holders, and a large mercurial barometer, borrowed from the Harvard Observatory, was pitched headlong into the sand.  Fortunately no damage was done, and after a lively chase the runaway mule was brought back by Corporal Gamarra.  After Mr. Hinckley was remounted on his dangerous mule we rode on for a while in peace, between cornfields and vineyards, over paths flanked by willows and fig trees.  The chief industry of Vitor is the making of wine from vines which date back to colonial days.  The wine is aged in huge jars, each over six feet high, buried in the ground.  We had a glimpse of seventeen of them standing in a line, awaiting sale.  It made one think of Ali Baba and the Forty Thieves, who would have had no trouble at all hiding in these Cyclopean crocks.