“_Hog-wallows._”—In the _field_ the wetting of cracked clay soils produces some very curious effects. The effect of the first light rains usually is to crumble off the edges or angles near the surface, the materials thus loosened falling into the lower portion of the cracks. This is repeated at each successive shower followed by sunshine, the crevices thus becoming partly filled with surface soil. When, subsequently, the heavier and more continuous rains wet the land fully, also causing the consolidated mass in the crevices to expand, the latter cannot close on account of the surplus material having fallen into them; the result being that the intermediate portions of the soil are compelled to bulge upward, sometimes for six or more inches, creating a very uneven, humpy surface, well-known in the southwestern United States as “hog-wallows.”[33] [33] A totally different kind of “hog-wallows,” occurring in California and the arid region generally, have been described in a previous chapter under the head of Aeolian soils (See chapt. 1, p. 9). Such a surface is always therefore an indication of an _extremely heavy soil, difficult to cultivate_; yet embracing some of the most highly and permanently productive lands known in the United States, and in India, where the “regur” lands of the Deccan are of this character; they have been cultivated without fertilization for thousands of years. The subjoined physical analyses of lands of such extreme character as to be almost uncultivatable will serve to exemplify their physical composition. PHYSICAL ANALYSES OF HEAVIEST CLAY SOILS.