[Illustration: FIG. 207.—Borax deposits upon the floor of Death valley, California (after a photograph by Fairbanks).] =Dry weathering—the red and brown desert varnish.=—In desert lands the fierce rays of the sun suck up all the available moisture, and the water table may be hundreds of feet below the surface. Roots of trees a hundred feet or more in length have been found to testify to the fierce struggle of the desert plant with the arid conditions. In humid regions the meteoric water dissolves the more soluble sodium salts near the surface of the rock and carries them out to the ocean, where they add to the saltness of the sea. In the desert the rare precipitations prevent an outflow, but the sun’s strong rays suck out with the moisture the salts from within the rock, and evaporating upon the surface, the salts are left as a coat of “alkali”, which is in part carried away on the wind and in part washed off in one of the rare cloudbursts. In either case these constituents find their way to the lowest depressions of the basin, where they contribute to the saline deposits of the desert lakes (Fig. 207). [Illustration: FIG. 208.—Hollowed forms of weathered granite in a desert of central Asia (after Walther).] Certain of the saline constituents of the rocks, as they are thus drawn out by the sun’s rays, fuse with the rock at the surface to form a dense brown substance with smooth surface coat, known as _desert varnish_. Within the interior a portion of the salts crystallize within the capillary fissures, and like water freezing within a pipe, they rend the walls apart. As a direct consequence of this disintegrating process the interior of rock masses may crumble into sand; and if the hard shell of varnish be broken at any point, the wind makes its entrance and removes the interior portion so as to leave a hollow shell—the characteristic “pocket rock” (Fig. 208) of the desert. The nummulitic limestone of Mokkatan and many of the great hewn blocks of Egyptian limestone sound hollow under the tap of the hammer, and when broken, they reveal a shell a few inches only in thickness (Fig. 209). [Illustration: FIG. 209.—Hollow hewn blocks in a wall in the Wadi Guerraui (after Walther).] The brown desert varnish is one of the most characteristic marks of an arid country. It is found in all deserts under much the same conditions, and is especially apt to be present in sandstone. When scratched, the surface of the rock becomes either cherry-red, indicating anhydrous ferric oxide, or it is yellowish, due to the hydrated iron oxide which we know as iron rust. Thus it is seen that the sands of deserts, in contrast to those yielded by other processes within humid regions, have a characteristic red color, and this may vary from brownish red upon the one hand to a rich carmine upon the other.