Page:The Incas of Peru.djvu/240

202 of the coast is a rainless desert traversed, at intervals, by fertile valleys.

The climate of the coast is modified and made warmer by another agency. Not only is the constantly prevailing wind from the south, there is also a cold current always flowing with a temperature several degrees lower than that of the surrounding ocean. It is believed by some to be derived from the Antarctic regions, by others that it is formed by cold water in the depths rising to the surface. Be this how it may, the Humboldt current, as it has been called since 1802, profoundly affects the climate of the Peruvian coast, which is cooler and drier than any other tropical region.

Although the greater part of the coast region consists of desert or of arid and stony ranges of hills, it is watered by rivers which cross the desert at intervals and form fertile valleys of varying width. The deserts between the river valleys vary in extent, the largest being upwards of seventy miles across. On their western margin steep cliffs rise from the sea, above which is the desert plateau, apparently quite bare of vegetation. The surface is generally hard, but on some of the deserts there are great accumulations of drifting sea sand. This sand forms isolated hillocks, called medanos, in the shape of a crescent, beautifully symmetrical, with sharp ridges, and their convex sides turned towards the trade-wind. Any stone or dead mule forms a nucleus for them; but they are constantly shifting, and a strong wind causes