20 
BULLETIN 61, U. S. DEPARTMENT OF AGRICULTURE. 
(late Quaternary) lava flows, which undoubtedly originated from fissures, 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 sulphur compounds are conspicuous in the gaseous and solid ejecta 
of volcanoes. Much of the chlorine that we find compounded with sodium undoubtedly 
originated from volcanic activity. 
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 return 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 
divided into three groups — undecomposed rock fragments, partially decomposed 
rock fragments, and products of complete 1 rock decomposition. The last group 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, muscovite, 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. 
Soluble . 
...do.... 
...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 
Magnesium 
Do. 
Insoluble. 
Do. 
It is evident 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 gypsum 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 gypsum. 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 be thrown down by calcium and magnesium. 
The analyses of waters coming from the zone of weathering invariably show small 
quantities of silica, ferric oxide, calcium carbonate, calcium sulphate, and alumina. 
i The word "complete" is used in a restricted sense. 
