E. W. Hilgard — Peculiarities of Rock - Weathering. 263 



of '036 mm , in the arid California soils as large an amount was 

 found in the sand-grain sizes between -12 and -50 mm as in the 

 •016 to •025 Ium silt of Mississippi. 



How does this radical difference between the sands of the 

 arid and humid regions come about? The pulverulent nature 

 of the soils formed under arid conditions supplies the answer. 

 The binding material, the " clay," that forms so important a 

 character of the prevalently clayey and loamy soils of the 

 humid region, is in the properly arid soils present in minute 

 proportions only ; except of course where clays from prexist- 

 ing clay formations have taken part in soil formation, or where, 



Per cent Phos- 



in phoric Soluble Total 



Sediments. Soil. Potash. Lime. Magnesia. Acid. Silica. Alumina, soluble. 



Colloidal clay, ? diameter. 



Mississippi 21-64 -32 -03 "29 -04 747 3*97 11 '82 



California (Chino) ... 760 -16 -14 -17 -04 1-70 1-35 3-56 



California (Jackson).. 16-43 -13 -12 -08 -05 2*83 2-13 5*34 



Very fine silt, -016 mra diam. 



Mississippi 35-10 -41 -15 -36 -07 2-87 1'36 5-22 



California (Chino) .. . 18'53 -24 -53 -29 '06 4-96 1-76 7'84 



California (Jackson) . . 34-90 -10 -04 -08 -02 2'50 2*44 5'18 



Fine silt, -016--025 mm diam. 



Mississippi 13-67 -12 -09 -10 "02 -32 -17 -82 



California (Chino) .. . 5-49 -05 -11 -02 -01 -80 -51 1-50 



California (Jackson).. 9-96 -08 -04 -10 -01 1-01 1-01 2*25 



Silt, -025--036 mm diam. 



Mississippi ... .. .. ..-36.. .. .. -36 



California (Chino) ... 3-92 .. _. _. ._ ... .. lost 



California (Jackson) . 7*68 -06 '02 -05 -01 -82 -74 1-70 



Silt, -036-047 mm diam. 



California (Chino) 6-40 -05 -18 -07 '01 -80 -55 1-66 



California (Jackson).. 8*21 -04 -01 -003 -001 -43 -64 1-22 



Coarse silt, •047-072 mm diam. 



California (Chino) ... 7"92 -06 -20 -03 -02 -89 -59 1-79 



California (Jackson).. 5-91 -03 -01 -01 -003 -42 -30 -77 



Fine sand, -072--120 mm diam. 



California (Chino) ... 11-87 -06 -26 -10 -03 -98 .. 1-43+ 



California (Jackson) . . 4-03 -01 -01 -005 -003 -28 -09 -408 



Coarser sand, -120--050 mm diam. 



California (Chino) ... 36-11 -11 -69 -12 -04 2'43 1-59 4-98, 



California (Jackson).. 10*10 __ .. .. notdetd. 



