=Some characteristics of the desert wastes.=—The great stretches of the arid lands have been often compared to the ocean, and the Bedouin’s camel is known as “the ship of the desert.” Though a deceptive resemblance for the most part, the comparison is not without its value. Both are closed basins, and it is in this respect that the desert and the ocean may be said to most resemble each other, for none of the water and none of the sediment is lost to either except as boundaries are, with the progress of time, transposed or destroyed. Flatness of surface and monotony of scenery both have in common, and the waters and the sand are in each case salt; yet the ocean, from the tropics to the poles, has the same salts in essentially the same proportions, while in the desert the widest variations are found both in the salts which are present and in their relative quantities. Upon the borders of the ocean are found ridges of yellow sand heaped up by the wind, but these ramparts are small in comparison to those which in deserts are found upon the borders (plate 7 A). The desert is a land of geographic paradoxes. As Walther has pointed out, we have rain in the desert which does not wet, springs which yield no brooks, rivers without mouths, forests preserved in stone, lakes without outlets, valleys without streams, lake basins without lakes, depressions below the level of the sea yet barren of water, intense weathering with no mantle of disintegrated rock, a decomposition of the rocks from within instead of from without, and valleys which branch sometimes upstream and sometimes down. Within the deserts curious mushroom-like remnants of erosion afford a local relief from the searching rays of the desert sun. Pocket-like openings large enough for a hermit’s habitation are hollowed out by the wind from the disintegrated rock masses. Amphitheaters open out from little erosion valleys or wadi, and isolated outliers of the mountains stand like sentinels before their massive fronts. Because of the general absence of clouds above a desert, no shield such as is common in humid regions is provided against the blinding intensity of the sun’s rays. Sun temperatures as high as 180° Fahrenheit have been registered over the deserts of western Africa. Every one is familiar with the fact that a blanket of thick clouds is a prevention of frosts at night, for, with the setting of the sun and the consequent radiation of heat from the earth, these rays are intercepted by the clouds, returned and re-returned in many successive exchanges. Over desert regions the absence of any such blanket of moisture is responsible for the remarkable falls of temperature at sunset. Though shortly before temperatures of 100° Fahrenheit or greater may have been measured, it is not uncommon for water to freeze during the following night. Much the same conditions of sudden temperature change with nightfall are experienced in high mountains when one has ascended above the blanketing clouds.