818 DESCRIPTIVE GEOLOGY, 



Truckee Range, extending from Desert Station to Pyramid Lake, a belt of 

 tufa terraces, showing the ancient water-levels of Lake La Hontan. These 

 are formed mainly of blocks and boulders of basalt, and of diabase in the 

 region of Diabase Hills, cemented together by a calcareous tufa, the tufa 

 also occurring as a compact mass. The highest of these tufa terraces in the 

 region of Wadsworth lies about 350 feet above the town, but there are 

 indistinct traces at considerable elevation above this level of other beach- 

 lines formed before the waters were dense enough to deposit any large 

 ■' amount of tufa. They are indicated partly by slight beach-lines and in 

 part by flat tabular lines of erosion in the rock. 



About 12 miles down the river from Wadsworth, these tufa terraces on 

 the east wall of the valley are quite thick, and the calcareous matter has 

 piled up over the boulders and upon the rounded protrusions of the old 

 shore to the thickness of 30 or 40 feet. Here are a number of pebble 

 beaches, showing a rough stratification, each coated with a thin covering of 

 tufa, cemented firmly to its neighbors by the same calcareous cement. 



Near the northwest point of the chain of hills which walls in the valley 

 on the east, this tufa formation develops itself in a well-defined stratum or 

 sheet extending over a wide area of the valley-mesa. It is in some places 

 a continuous bed, but for the most part occurs in ellipsoidal masses from 4 

 to 8 feet in diameter. The upper surface is quite smooth and very compact, 

 but around the hollow underneath it is a mere network of crvstalline frao-- 

 ments, tangled together and re-cemented by a further deposition of carbonate 

 of lime. Among the forms of these detached masses are globes and flattened 

 spheres, but more commonly a mushroom form, with a distinct stem pene- 

 trating the sands. There is one large area where these mushroom forms 

 are seen lying as closely together as they can stand, the intervening 

 spaces being occupied by a mixture of tufa and sand. These curious 

 fantastic forms have frequently been mistaken for some variety of coralline 

 growth. 



Throughout this lime deposit are many small fresh-water shells. In 

 the loose dry soil of the valley-mesas may be found an immense quantity of 

 minute ostracoid shells, so light and delicate that they are drifted about by 

 the prevailing winds, frequently carried long distances and strewn over the 



