SOILS. 



the fields; the surface of which, apart from the cracks usually 

 formed, mav be several inches lower in the drv season than 



during wet weather. 



RED Ct \v -.nt BLACK " ADORE " CI.AY SOIL. 



FlG 13 Expansion on \Vetting and Contraction on Drying ot heavy <.!.iy soils. 



Contraction <>n U'cttiiig. In the ca-e of alkali soils contain- 

 ing much carhnirite of -"'la, a very notable ctinti\icti<>n occurs 

 in netting the loose, dry -oil. The cause is here obviously the 

 eolhip-e of the crumbs, formed in dry tillage or crushing, into 

 single grain-, closely parked. The same result i- ob-erved in 

 the naturally depressed "alkali spots" (see chapt. 22). 



Hog-wallows." -In the held the wetting of cracked clay 

 soils produce^ some very curious effects. The effect of the 

 first light rains usually i- 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 succe--- 

 ive shower folloued by sunshine, the crevices thu- becoming 

 partly rilled with surface -oil. 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 portion- of the soil 

 are compelled to bulge upward, sometimes for six >r more 

 inches, creating a very uneven, humpy surface, well-known in 

 the southwestern United States a- " hog-wallows." l 



1 A totally different kind of " IHIL; wallow-." > 'irrinij in California and the arid 

 region generally, have been described in a previous chapter under the head of 

 Aeolian soils (See chapt. i. p 



