LOWER TRUCKEE VALLEY. 819 



desert like ordinary sands and gravels. They have not as yet been care- 

 fully studied, and their specific characters are unknown. 



Another feature of the desert in the region of Wadsworth, %Yhere they 

 never could have been brought by erosion, are large numbers of basaltic 

 bombs, characterized by their general spherical form, some of which are as 

 vesicular as a pumice, the whole body of the bomb being made up of the 

 tliinnest possible basaltic skeleton. Others again consist of dense, compact 

 rock. 



A specimen of the tufa from the Truckee Valley has been chemically 

 examined by Mr. E. W. Woodward, who reports as follows : 



Alumina 0.89 0.86 



Lime 49.77 49.80 



Magnesia 3.28 3.25 



Soda 0.79 0.88 



Potassa 0.15 0.20 



Carbonic acid 41.02 40.94 



Sihca 3.01 3.10 



Water 1.41 1.40 



Phosphoric acid trace trace 



Sulphuric acid trace trace 



100.32 100.43 



It agrees closely with the analysis of similar tufas found at Pyramid 

 Lake and on the Carson Desert. 



Owing to the extreme softness of the Pliocene strata, erosion does not 

 result in the forms of spires and pinnacles so common in the eastern Ter- 

 ti-ary basins, but rather in rounded, terraced buttresses, separated from each 

 other by sharp re-entering alcoves. 



Where the wall of basalt comes down from the north,, on the east side 

 of the valley, the river cuts a narrow sharp passage in the Pliocene, and in 

 the canon-bottom is exposed the top of an underlying volcanic mass, which 

 probably belongs to the purple dacites of the Virginia Range, and which 

 evidently suffered very sharp irregular erosion before the laying-down of 

 the Pliocene strata, since they enter and fill up all its carved-out depressions, 



V. 



