PYRAMID LAKE. 823 



means of the pocket-level were made of these ; the highest and most sharply 

 defined was found to be 498 feet above the lake, the others standing 

 respectively at 324, 259, and 9 7 feet above the present water-mark. As the 

 low divide of volcanic hills between Pyramid Lake and the Mud Lakes 

 lies below the higher elevations of the ancient lake, it is evident that the 

 waters of the former were at one time connected with the broad open waters 

 to the north. Incrustations of calcareous tufas, deposited by the receding 

 waters, line the shore, adhering to the rocks, in broad bands frequently two 

 and three feet in thickness. Many large masses of rock are so completely 

 covered with tufa as to suggest, at first sight, a solid mass of carbonate of 

 lime, formed by thermal action around some hot spring. 



The pyramid which gives the name to the lake is a tufa-encrusted 

 island, rising about 400 feet above the level of the lake, and has a warm 

 spring about half-way up its slopes, while the " Tufa Domes" are large 

 botryoidal-shaped masses of tufa, from 50 to 60 feet in height, extending 

 out from the shore in the direction of the pyramid, and in one case quite 

 isolated, as seen in the view presented in Plate XXV. The terraced 

 beaches are also indistinctly brought out on the pyramid. 



These calcareous tufas have a light-brown color, and vary from a com- 

 pact homogeneous rock to a rough porous entangled mass of rudely formed 

 crystals. An analysis of tufa, taken from one of the "Tufa Domes", yielded 

 Mr. R. W. Woodward the following : 



Lime 47.27 47.48 



Magnesia.. 2.89 2.50 



Alumina 2.14 2.54 



Soda :.. 0.51 0.48 



Potassa : 0.22 0.19 



Carbonic acid 38.23 38.52 



Silica 7.27 6.90 



Phosphoric acid trace trace 



Sulphuric acid trace trace 



Water 1.20 1.20 



99.73 99.81 

 The silica occurs both combined and as free sand. 



