20 J GEOLOGICAL HISTOIIY OF LAKE LAHONTAN. 



a photograph of a section of a small dome or loaf-like mass, the top of 

 which has been removed b}' weathering. 



As the lake stood about 220 feet above the thinolite terrace when the 

 highest deposit of dendritic tufa was formed, the geographical distribution 

 of this variety is of broader extent than that of the thinolitic; but it is not 

 so widely spread as the lithoid tufa. 



It not only occurs in far greater abundance than either of the other 

 varieties, but is found in various relations to the lacustral sediments, etc., 

 which tbe other tufas have not been observed to present. In many locali- 

 ties on the deserts where nuclei of gravel, shells, etc., were present, the sur- 

 face is frequently covered here and there with tufa-growths having a strik- 

 ing resemblance to mushrooms or tuberous roots, which are frequently 

 spoken of as " fossil mushrooms," "fossil turnips," etc. A few characteristic 

 examples of the pseudo-vegetable forms are represented on Plate XXXVII. 

 Fig. A shows a growth about four inches high springing from a water- worn 

 pebble; Fig. B, of one-third natural size, is a deposition of tufa on a rOcky 

 crag that projected an inch or two above the lake bottom; in Fig. C the 

 deposition commenced about grains of sand; Figs. D and E show the 

 tops of mushroom-shaped growths. At some localities the "mushrooms" 

 occur of large size and form a continuous pavement over the surface of the 

 former lake bottom, as may be seen along the road between Wadsworth 

 and Pyramid Lake, and again in the Carson Canon, north of Churchill 

 Valley. At these localities the symmetrical outline of the tufa-forms has 

 frequently been modified by the contact of one with another during their 

 formation, so- that now they are polygonal instead of circular in horizontal 

 section. These masses are frequently hexagonal when seen from above and 

 have rounded dome-shaped tops, which are sometimes hollowed out by the 

 weathering of their sunmiits and cellular interior in such a manner as to 

 form irregular vases. In many instances the separate mushroom-like masses 

 springing from independent nuclei are 10 or 15 feet in diameter and weigh 

 many tons. In the walls of the canons carved b}' the Humboldt and 

 Truckee rivers through the sediments of Lake Lahontan sections are ex- 

 posed of continuous layers of dendritic tufa, interstratified with marls and 



