J 52 GEOLOGICAL HISTORY OF LKAE LAHONTAN. 



overplaced along the shoreward margin by similar material that has been 

 swept down from the slopes above. Sometimes the marls are deeph' eroded 

 and present typical " bad land " topography in miniature. 



The White Terrace may be seen at a number of places about the north- 

 ern end of Pyramid Lake, and in the pass leading to Honey Lake Valley. 

 At the southern end of Smoke Creek Desert it was again observed, with its 

 normal elevation of 320 feet above Pj^ramid Lake. Further northward, it 

 occurs at a number of localities on the steep borders of the Black Rock 

 Desert. From the numerous exposures observed it is evident that this 

 terrace occurs all about the deeper portions of Lake Lahontan, and may be 

 considered as co-extensive with the dendritic terrace with which it coincides 

 in elevation. The occurrence of the marl as a shore deposit is independent 

 of the character of the rock against which it rests ; it occurs with equal pu- 

 rity on alluvial slo.pes and on shores of limestone, basalt, rhyolite, etc. It 

 is found in abundance about isolated buttes that formed small islands in the 

 former lake, as well as along the shores of the mainland, and is therefore 

 evidently not a product of erosion. Occasionally, however, the marl is min- 

 gled with sand and pebbles, and when it takes the form of a free bar the 

 proximal end will be found to contain more foreign material than the distal 

 extremity, thus indicating the assorting power of the currents that trans- 

 ported the material. 



At all the numerous localities where the White Terrace is exposed it is 

 composed of fine, incoherent, chalky marl, which is often richly chai-ged 

 with the shells of fresh-water mollusks. Li places the deposit is 40 or 50 

 feet thick, and homogeneous throughout. An analysis of a typical sample 

 collected on the western border of Pyramid Lake Valley, as reported by 

 Dr. T. M. Chatard, is given below, and shows that the material is essentially 

 an impure calcium carbonate containing a high percentage of silica : 



Water (HiO) 3.32 



Calcium carbonate (CaCOa) 64.82 



Silica (SiO.2) 22.00 



Alumina (AI.2O3) 5.14 



Iron (FcjOa) 2.04 



Liiue(CaO) 0. M 



Magnesia (MgO) 1.89 



100. 14 



