208 GEOLOGICAL HISTORY OF LAKE LAHOSTAN. 



with the mode of their formation These tufa-forms occur of all sizes, from 

 mushroom-shaped cakes a few inches high, up to castellated masses rising 

 over a hundred feet above the desert, and frequently contain many thousands 

 of tons of calcium carbonate. IsoTated tufa-masses may be studied to advan- 

 tage about the shores of Pyramid and Winnemucca lakes, and at a few local- 

 ities on the borders of the Carson Desei't. The rugged promontory known 

 as the Needles is suiTOunded by a number of peculiarly shaped islands 

 which rise from 15 or 20 feet of water to a height of 40 or £0 feet above 

 the lake's surface. The Needles and all the associated islands are composed 

 of tufa, which takes the form of towers and domes of the most rugged and 

 picturesque description. The highest of the Needles rises like a cathedral 

 spire to the height of 300 feet, and is apparentlj^ composed of tufa throughout. 

 At the top only lithoid tufa is found ; at the base of the spire, where the rock 

 swells out and forms a dome, there is a great thickness of the dendritic vari- 

 ety ; at the base, where the rock has been weathered and broken, a heavy de- 

 posit of thinolite crystals is exposed, interstratified between the lithoid and 

 the dendritic. The precipitation of calcium carbonate has been so abundant 

 at this locality that the rocky nucleus about which the crystallization com- 

 menced can only be seen in a few places. 



In some instances tufa domes and towers are grouped in clusters and 

 unite with one another in such a manner as to form castle -like masses of 

 great size, which call to mind the ruins of mediseval strongholds. The ap- 

 pearance of one of these water-built structures standing on the western 

 shore of Pyramid Lake is shown on Plate XL ; like all the tufa deposits 

 in the basin that are not submei'ged, this ancient castle is fast crumbling 

 into decay and ruin. 



Isolated towers and shafts of tufa are sometimes seen standing on the 

 desert in independent masses that are frequently 5U or GO feet high, and 

 furnish most instructive examples of deposits of this nature. These struct- 

 ures are frequently weathered and broken in such a manner as to expose 

 every desirable section of their interiors, and afford abundant opportunity 

 for the study of their anatomy. The inspection of some of these broken 

 shafts shows at a glance that they have a concentric structure, and are com- 

 posed of three varieties of tufti, as in the case of the deposits sheathing the 



