104 E. W. Hilgard — Geologic Efficacy of 



practically free from lime) with standard acid. The outer 

 curves represent the amount of hydrocarbonate formed in each 

 case, the inner ones that remaining after hot evaporation. 

 While the intervals between the successive points of obser- 

 vation are wider than could be desired, there can be no 

 doubt as to the general outcome. It is seen that up to \\ 

 grams per liter, the curves for sodium and potassium almost 

 coincide, within the limits of errors of observation : from that 

 point, however, they rapidly diverge, the weight of the sodium 

 salt formed becoming less than that of the potassium salt, as 

 would be anticipated from the difference in atomic weight. 

 At 8 grams per liter the respective amounts of the two salts 

 formed are almost exactly in the proportion of their molecular 

 weights and so continue to the limit of the experiments (ten 

 gj*ams). 



It will also be noted that up to one gram per liter in the 

 case of the potassic salt, and to 8 grams per liter in the case 

 of the sodic, the entire amount of alkali sulphate is transformed 

 into carbonate. Beyond that point there is a rapid decrease in 

 the proportion of sulphate transformed ; so that at 8 grams 

 per liter only about one-fourth suffers the change, while at the 

 came time the absolute amount formed has almost ceased to 

 increase. The existence of this limit prevents this simple 

 mode of producing sodic carbonate from being technically 

 useful, as it would not pay to evaporate such dilute solutions. 

 Nature, however, has, and still continues, to perform this ser- 

 vice to humanity in the arid regions, from which Trona, Urao 

 and Kara have for ages been imported into Europe, forming in 

 former times the only source of sodic carbonate. In these 

 crude commercial articles the chloride and sulphate is invaria- 

 bly associated with the carbonate, to a greater or less degree, 

 indicating plainly the process and the materials concerned in its 

 formation. 



As to the calcic carbonate : Calcareous rocks are known to 

 exist in the African and Mexican localities from which the 

 respective products (Trona and Urao) are derived. But there 

 is a wider cause for the almost universal presence of the calcic 

 carbonate in the surface formations of the arid regions, which I 

 have discussed elsewhere ;* a fact confirmed by either direct or 

 casual observation of travelers and explorers in those regions, 

 all over the world. The almost universally calcareous nature 

 of the soils covering the region west of the Rocky Mountains, 

 irrespective of the nature of underlying rocks or adjacent 

 mountains, has now become a matter of common note. 



Alkali soils. — The phenomena just discussed afford an insight 



* See Bulletin No. 3, U. S. Weather Bureau, 1892 ; Wollny's Forschungen. etc., 

 1893; Annales de la science agronomique, 1893; and Reports of the California 

 Experiment Station, 



