14 



BULLETIN 61, U. S. DEPAKTMENT OF AGRICULTUEE. 



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 : Rhyolitea 

 and granites; andesite basalts, diabases, and diorites; metamorphics, 'limestones, 

 and sedimentary and water areas. 



Areal distribution of rocks in the Great Basin region. 



Rock. 



Truckee 

 quad- 

 rangle. 



Fortieth Parallel 

 Survey Atlas. 



No. 5. 



Nos. 4 

 and 5. 



Ball's 

 map of 



south- 

 west 

 Nevada 

 and east- 

 em Cali- 

 fornia. 



Rhyolite and granite 



Basalt, diabase, and diorite. 



Metamorphic 



Limestone 



Sedimentary and water 



22.0 

 45.1 

 6.15 



18.8 

 14.2 



»22.5 

 25.5 



21.5 

 11.5 



26.8 



67.0 



53 



12.1 

 54.9 



I 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 Truckee 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 Truckee 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 iaken 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 witn 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 important types of 

 igneous rocks is given in Table X (Appendix). From the figures in this table the 

 following table has been calculated : 



