20 



BULLETIN 61, U. S. DEPAKTMENT OF AGRICULTUEE. 



(late Quaternary) lava floTVS, which undoubtedly originated from fissui'es, are to be 

 noted . 



While we have no evidence at present as to the amount and kind of saline material 

 in the ejecta of these cones and fissures, from our knowledge of volcanic eruptions at 

 present taking place we must conclude that a considerable part at least of the saline 

 material at present in the basin came from volcanic sources. Chlorine, sulphuric acid, 

 chlorides, and sulphm* compounds are conspicuous in the gaseous and solid ejecta 

 of volcanoes. Much of the chlorine that we find compounded with sodimn undoubtedly 

 originated from volcanic acti^^.ty. 



ATMOSPHERE. 



From the atmosphere, important contributions of carbonic acid gas and, to some 

 extent, chlorine, chlorides, and nitrogen compounds are being made. Wind erosion 

 is undoubtedly responsible for the retmTi of some of the saline material from the playas 

 to the mountain ranges. 



REACTIONS IN THE ZONE OF WEATHERING. 



REACTIONS OF SOLUTION. 



The products of disintegration and decomposition in the zone of weathering may be 

 di-vided into three groups — undecomposed rock fragments, partially decomposed 

 rock fragments, and products of complete^ rock decomposition. The last gi'oup may 

 be divided into soluble and insoluble products. Of these the former would consist 

 of alkalies and alkaline earths, together with acid radicals, chlorine, sulphuric anhy- 

 dride, carbonic and bicarbonic, nitric, boric, and phosphoric; the latter would 

 consist of kaolinite, musco-vite, quartz, talc, zeolitic minerals, limonite, calcite, and 

 chlorite. Between the alkalies and alkaline earths and the acid radicals certain 

 important reactions would take place. The relative abundance and kind of bases 

 and acid material would .determine these reactions. The solubilities of the more 

 important constituents are given, in-the following table; 



Solubility of important constituents of decomposition product. 



Basic element. 



Chloride. 



Sulphate. 



Carbonate. 



Bicarbon- 

 ate. 



Nitrate. 



Borate. 



Phosphate. 



Sodium 



Soluble . 

 ...do.... 

 ...do 



Soluble... 



...do 



Insoluble.. 



Soluble.... 



Soluble... 



...do 



Insoluble.. 



...do 



Soluble... 



...do 



Soluble to 

 slight 

 extent. 

 ...do 



Soluble . 

 ...do.... 

 ...do.... 



...do 



Soluble . 

 ...do.... 



Slightly 

 solu- 

 ble. 



I n solu- 

 ble. 



Soluble. 



Potassium •. 



Calcium 



Do. 

 Insoluble. 





do 



Do. 













It is e^'ident that in a system consisting of all chlorides and nitrates of the bases 

 named all of the compounds would be soluble and only in the case of their concen- 

 trated solution would any salts separate out. It is apparent that the latter condition 

 would rarely be present in the zone of weathering. In a system of chlorides, sulphates, 

 carbonates, bicarbonates, nitrates, borates, and phosphates of sodium and potassium 

 no reactions resulting in insoluble compounds could take place. In a system of 

 chlorides and sulphates, lime would be the only base precipitated as a sulphate. 

 This is indicated by the fact that gj^^sum is not an infrequent mineral in the zone 

 of weathering, and its presence is no doubt due in part to reactions of this nature. 

 The most common system that we find includes the chlorides, sulphates, carbonates, 

 and bicarbonates of sodium, potassium, calcium, and magnesium. In this system 

 calcium and magnesium would be thrown down as comparatively insoluble carbonates 

 and calcium also as gj-psum. We would expect the solution resulting to contain 

 chlorides, sulphates, carbonates, and bicarbonates of sodium and potassium. In a 

 system in which calcium and magnesium predominate we would expect to find 

 mainly soluble chlorides, as all carbonic acid and sulphuric acid, except that required 

 to saturate the system, would Vje thrown down by calcium and magnesium. 



The analyses of waters coming from the zone of weathering iuAariably show small 

 quantities of silica, ferric oxide, calcium carbonate, calcium sulphate, and alumina, 



1 The word "complete" is used in a restricted sense. 



