262 E. W. Hilgard — Peculiarities of Rock -Weathering. 



the almost invariable high productiveness of the desert sands 

 under irrigation is becoming a somewhat familiar conception, 

 yet accepted with difficult} 7 because in the more familiar humid 

 region, " poor sandy lands " are a well-authenticated fact, and 

 a " strong " or " substantial " soil is one containing a more or 

 less considerable proportion of clay. 



The reasonableness of this popular idea is well illustrated in 

 the investigation made years ago by Dr. R. EL Loughridge,* 

 of a very generalized loam soil covering the uplands of Ken- 

 tucky, of Tennessee, Mississippi, and Louisiana east of the Mis- 

 sissippi river. In this investigation it was shown that by far the 

 greater portion of the recognized plant-food ingredients was 

 contained within the impalpable portion, viz., the " clay " and 

 finest silts, only an insignificant amount being found in the 

 " sandy " portion of the soil. The extreme poverty of the 

 sandy lands of Florida, as shown by analyses subsequently 

 made by him, fully corroborated these points for the humid 

 region of the cotton states at least. And in Europe, the 

 sandy lands of the northern plain are equally sterile without 

 fertilization. 



How is it then that the sands and dust of the arid region 

 are so highly productive so soon as irrigated ? 



A comparative microscopic examination of sands from Flor- 

 ida and that from the arid deserts at once reveals the difference, 

 which is equally accentuated by their chemical analysis. While 

 the Florida or Mississippi soil sand under the microscope shows 

 almost wholly quartz grains with clean, polished surfaces, the 

 typical desert sand shows a great variety of minerals in granu- 

 lar form, coated with half-decomposed, finely pulverulent 

 mineral matter, which also constitutes the " dust " portion of 

 the material. Analyses made of the coarse and fine portions 

 of such soils by Mr. L. M. Tolman f and by E. C. Lea (unpub- 

 lished), proved that the surface-covering of the coarse sand 

 grains was practically of the same composition as the fine dust 

 itself. The subjoined table shows that in the typical upland 

 clay loam from Mississippi (a soil noted for its high product 

 of cotton) dissolution of soil ingredients substantially ceased 

 when a diameter of only - 036 mm of the soil granules was reached ; 

 while from that limit, in the coarser portions of the California 

 soils, obtained by the same process of hydraulic eleutriation, 

 and up to half a millimeter diameter, there was not only no 

 diminution but an actual increase of acid-soluble matters. 



It thus appears that while in the humid Mississippi soil, solu- 

 bility of plant food practically ceased above a grain-diameter 



* This Journal, Jan. 1874. 



f Eep. Calif. Exper. Station for 1898-1901, p. 33. 



