coastal desert. One reads so much of the great tropical
jungles of South America and of wellnigh impenetrable forests that
it is difficult to realize that the West Coast from Ecuador, on
the north, to the heart of Chile, on the south, is a great desert,
broken at intervals by oases, or valleys whose rivers, coming
from melting snows of the Andes, are here and there diverted for
purposes of irrigation. Lima, the capital of Peru, is in one of the
largest of these oases. Although frequently enveloped in a damp fog,
the Peruvian coastal towns are almost never subjected to rain. The
causes of this phenomenon are easy to understand. Winds coming from
the east, laden with the moisture of the Atlantic Ocean and the
steaming Amazon Basin, are rapidly cooled by the eastern slopes of
the Andes and forced to deposit this moisture in the montana. By
the time the winds have crossed the mighty cordillera there is no
rain left in them. Conversely, the winds that come from the warm
Pacific Ocean strike a cold area over the frigid Humboldt Current,
which sweeps up along the west coast of South America. This cold belt
wrings the water out of the westerly winds, so that by the time they
reach the warm land their relative humidity is low. To be sure, there
are months in some years when so much moisture falls on the slopes
of the coast range that the hillsides are clothed with flowers, but
this verdure lasts but a short time and does not seriously affect the
great stretches of desert pampa in the midst of which we now were. Like
the other pampas of this region, the flat surface inclines toward the
sea. Over it the sand is rolled along by the wind and finally built
into crescent-shaped dunes. These medanos interested us greatly.

The prevailing wind on the desert at night is a relatively gentle
breeze that comes down from the cool mountain slopes toward the
ocean. It tends to blow the lighter particles of sand along in a
regular dune, rolling it over and over downhill, leaving the heavier
particles behind. This is reversed in the daytime. As the heat
increases toward noon, the wind comes rushing up from the ocean to
fill the vacuum caused 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 a