190 GEOLOGICAL HISTOKY OF LAKE LAOONTAN 



especially favorable for the deposition and preservation of calcareous de- 

 posits. The tufa frequently forms a sheathing 50 or 60 feet thick upon the 

 older rocks; at other times it assumes the form of domes and castellated 

 masses that in some casea^ rise a hundred feet above the nuclei about 

 which crystallization first took place. Of all such localities in the basin 

 the most remarkable are the Marble Buttes at the southern, and the Nee- 

 dles at the northern end of Pyramid Lake; Anaho and Pyramid islands 

 are also loaded with immense accumulations of tufa, and are points of spe- 

 cial attraction to the student of chemical geology. 



Early in the examination of these deposits it was found that they occur 

 in definite layers, and form a well-defined sequence in which three main 

 divisions, together with many minor variations, may be easily traced 

 Classifying the major divisions according to the structure of the rock, begin- 

 ning with the first formed, we have: (1) Lithoid tufa; (2) Thinolitic tufa; 

 (3) Dendritic tufa. 



LITHOID TUFA. 



We have applied this name to the first of the three main deposits of 

 calcium carbonate precipitated from the waters of Lake Lahontan. It is 

 compact and stony in structure, light yellowish gray in color, and weathers 

 into forms of extreme ruggedness. It frequently shows a concentric and 

 sometimes a well-marked tubular structure when seen in cross-section, but 

 when forming a coating to rock surfaces it is usually composed of thin, 

 superimposed layers. In well-exposed sections of lithoid tufa the banded 

 structure of the deposit is often distinctly marked, and at times, particularly 

 near the base of the deposit, or near the center of the dome-shaped masses 

 occurring in isolated positions, the layers of tufa having a compact stony 

 structure are separated by others of dendritic tufa, the character of which 

 will be described a few pages farther on. Viewed externally, the coatings 

 of this variety of tufa on cliffs and buttes appear to be formed of comb-like 

 masses, imbincated in such a manner as to resemble a massive thatch. This 

 appearance can only be seen to advantage above the limit of the more 

 recent tufas; it is especially well displayed on the upper portions of Anaho 

 Island ;ind the Marble Buttes. Much of the gravel forming the prelacustral 

 alluvial slopes of the Lahontan basin, as well as that composing the earlier- 



