14 
BULLETIN 61, U. S. DEPARTMENT OF AGRICULTURE. 
Nevada and eastern California. The areas represented by each formation were 
measured by a planimeter and the proportion of the whole area determined. The 
results of these measurements are given in Table IX. (See Appendix.) In the follow- 
ing summary the various volcanic and Plutonic rocks have been grouped : Rhyolites 
and granites; andesite basalts, diabases, and diorites; metamorphics, limestones, 
and sedimentary and water areas. 
Areal distribution of rocks in the Great Basin region. 
Rock. 
Truekee 
quad- 
rangle. 
Fortieth Parallel 
Survey Atlas. 
No. 5. 
Nos. 4 
and 5. 
Ball's 
map of 
south- 
west 
Nevada 
and east- 
ern Cali- 
fornia. 
Rhyolite and granite 
Basalt, diabase, and diorite. 
Metamorphic 
Limestone 
Sedimentary and water 
22.0 
45.1 
6.15 
18.8 
14.2 
122.5 
25.5 
21.5 
11.5 
26.; 
67.0 
53 
12.1 
54.9 
1 Assuming area of rhyolite and trachyte is one-half rhyolite and one-half andesite. 
The total areas occupied by igneous rocks are as follows: For the Truekee sheet, 67 
per cent; for map No. 5 of the Fortieth Parallel Survey, 33 per cent; for maps Nos. 
4 and 5 together, 48 per cent; for Ball's map, 33 per cent. The Truekee sheet may 
be considered as descriptive of an area in which igneous rocks dominate. This area 
would not be a fair representation of the whole basin region. The results obtained 
from the other three measurements indicate a range of 33 to 48 per cent. Which of 
these two measurements could be taken as representative of the basin region as a 
whole is a matter of doubt. Probably 40 per cent would be a fair figure to indicate 
areal distribution of igneous rocks in the basin region. This would leave 60 per cent 
for sedimentary and alluvial formations. On this basis some 84,000 square miles of 
the Basin region is occupied by igneous rocks. We may assume that acid rocks take 
somewhat less than one-half of this area and basic rocks somewhat more than one-half. 
The chemical composition of the rocks of the basin region has been determined by 
averaging the reported analyses of the various rocks. Table X (Appendix) gives the 
results of this study. 
SOURCES OF SALINES. 
The salines of the basin region consist of mixtures of chlorides, sulphates, carbonates, 
bicarbonates, nitrates, and borates of sodium, potassium, calcium, and magnesium. 
Lithium, alumina, ferric oxide, silica, bromine, iodine, phosphoric and arsenious 
acids have been detected in small amounts in the brines and waters of the basin. 
Alumina, ferric oxide, and silica are almost invariably found in small amount in river 
and lake waters and associated with saline crusts. Spectroscopic examination shows 
lithium in small quantity to be widely associated with saline material. 
Salines result from the disintegration and decomposition of igneous and sedimentary 
rocks, from the decomposition of alluvial and detrital fills, and from the waters of 
springs of deep-seated origin. During Quaternary times the basin region was the 
scene of numerous volcanic eruptions. How important these were as contributors to 
the salines can not now be told, but they must have been not unimportant sources of 
saline material. 
IGNEOUS ROCKS. 
In a previous section it has been shown that approximately 40 per cent of the basin 
area is covered by igneous rocks, and that somewhat less than one-half of this area is 
represented by rocks of an acid type, while somewhat more than one-half is repre- 
sented by rocks of a basic type. The composition of the more importarjt types of 
igneous rocks is given in Table X (Appendix). From the figures in this table the 
following table has been calculated: 
