1G8 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



quent deposition of the salts held in solution, which act as a cement and 

 sometimes change a loose debris heap to a compact conglomerate or breccia. 

 Subaerial deposits of this nature are common throughout the arid region of 

 the Far West. 



OOLITIC SAND. 



The presence of oolitic sand on the shore of Pyramid Lake has already 

 been referred to iu connection with the general description of the lake. 

 This material is evidently now forming, and in places has been cemented into 

 a compact oolite by the deposition of a paste of calcium carbonate between 

 the grains, and forms irregular layers several inches in thickness that slope 

 lakewards at a low angle. The oolitic grains composing the beach sands 

 are frequently a quarter of an inch or more in diameter, and would, per- 

 haps, more properly be designated as pisolite. When examined in thin 

 sections under the microscope each grain is seen to be made up of a large 

 number of concentric layers of calcium carbonate surrounding a particle 

 of sand or other foreign body which furnished the original nucleus. The 

 spherical form of the grains and the uniform thickness of the concentric 

 layers evidently indicates that the kernels were in motion during the slow 

 deposition of the concentric shells of which they are principally composed. 

 Oolitic sands occur also at a number of localities near the base of the den- 

 dritic tufa, thus indicating that the conditions necessary for the formation 

 of a deposit of this nature were then prevalent +hi-oughout the entire area 

 covered by Lake Lahontan. That the chemiv ttl conditions favoring the 

 formation of oolitic sands vary through wide limits is shown by the fact 

 that they are now forming both in Pyramid Lake and in Great Salt Lake. 

 The former contains less than half of one per cent, of solids in solution, 

 while the latter has varied from over twenty-two to about thirteen per cent, 

 during the past twenty years.'^ 



SURFACE MARKINGS. 



The surfaces of lacustrine deposits wlien laid bare and subject to des- 

 iccation become covered with a net-work of shrinkage cracks and are not 

 infrequently impressed with the foot-prints of animals; sometimes, too, the 



^^ See table of analyses at C, page 180. 



