40 BULLETIN 61, U. S. DEPARTMENT OP AGEICULTUEE. 



granite and Quaternary and recent alluvium surround the marsh. ^ The climatic 

 conditions are arid. As a result of the examination of the material obtained from 

 bores placed at a number of points on the marsh, Dole describes the structure in the 

 following words: 



"Brown mud 5 to 20 feet deep forms the upper layer of the marsh. Because of the 

 intense heat the surface of this mud is usually baked dry and hard enough to support 

 the weight of teams. Small scattered tracts have become dry enough to be pulver- 

 ulent for a depth of 1 to 2 feet, but over the greater part of the playa 4-foot holes are 

 sufficiently deep to strike soft mud. As this layer is composed of very small particles 

 and contains a large proportion of clay, the strong salt waters in it circulate very 

 slowly. The mud contains a great quantity of salt, though the crystals are small. 

 The brines obtained from it are very strong, and the surface is generally covered to a 

 depth of one-eighth to one-quarter of an inch by a wliite crust of salt that has crys- 

 tallized from solutions drawn to the surface by capillarity. 



"The upper mud along the west shore of the playa, particularly west of the 'islands, ' 

 contains nodules of calcareous tufa, wluch apparently have been formed by deposition 

 of calcium carbonate from the hard waters percolating into the marsh from Mineral 

 Ridge. The record of boring No. 13 shows that clay under the mud west of the 

 'islands' is underlain by white tufaceous materials, but no salt occurs at a depth less 

 than 41 feet except that in the abundant weak brines. 



"Well-defined beds of clay containing crystals of gypsum were penetrated east of 

 Goat 'Island' in borings Nos. .3 and 6, and these are underlain by beds of crystallized 

 salt containing saturated brine. Very stiff black, blue, red, gray, and brown clays 

 underlie the beds of salt or mixed salt and clay in boring No. 3 to a depth of 55 feet, 

 but in boring No. 6 the clays are interrupted by a stratum of gypsum-bearing clay 

 below the salt and a 6-inch stratum of salt at 47 feet, below which clay was again 

 encountered. 



"Except a shallow bed of light-gray calcareous material at 16 feet nothing but clay 

 containing weak brine was struck to a depth of 40 feet in boring No. 14, at the south 

 end of the playa. 



"Borings Nos. 11 and 12 indicate that the beds of salt in the northeastern part of the 

 marsh are denser than those farther south. The mud is underlain by clay and that 

 in turn by crystallized salt so hard that it has to be drilled. A much harder formation, 

 probably calcareous. tufa, was struck below the salt in both borings at a depth of about 

 36 feet. 



"The data afforded by the six deeper borings lead to the conclusion that the north- 

 eastern two-thirds of the playa is underlain at a depth of about 20 feet by beds of 

 crystallized salt, 5 to 15 fett tliick, mixed with more or less clay. It is doubtful if 

 deposits of so great extent occur west of Goat 'Island' or south of Alcatraz 'Island.' 

 Besides these beds practically all other strata to a depth of 50 feet contain appreciable 

 proportions of salt that readily dissolves in water percolating through them. " 



It is to be regretted that a greater niunber of borings were not made and the structure 

 of the marsh more accurately determined a\, greater depths over the northern half of the 

 basin. In a crude way the structure indicates the following cycle of events: 



1. The formation of a lake of unknown depth, but with a surface level below the 

 present level of the marsh. Salines accumulated in this lake, and a general silting up 

 took place. The area of this lake could not have extended much over two-tliirds of 

 the area of the present marsh. 



2. A period of desiccation in which a thin bed of salt was deposited at a 'depth of 

 about 48 feet below the present surface. Complete desiccation may not have taken 

 place but concentration sufficient to produce a saturated solution and the crystalliza- 

 tion of some of the sodium chloride (bore hole No. 6 shows one-half foot ^of crystals of 

 salt at 47.5 feet depth). Silts and clays were deposited and covered the layer of salt 

 crystals. Increased rainfall prevented further deposition of salt by diluting the lake 

 waters. 



3. The silting up of the lake continued to a level within 38 feet of the present surface. 

 Through the latter part of this period desiccation and consequent concentration of the 

 lake water proceeded simultaneously with the silting up. 



4. At tliis time the concentration of the lake was sufficient to precipitate gypsum 

 (bore hole No. 6, 38 feet, shows gray clay containing gypsum crystals). A period of 

 desiccation followed, and a salt bed was gradually built up reaching a position from 

 29 to 31 feet below the present level and a thickness of 6 to 7 feet. A period of 

 interruption followed, and then further deposition of salt took place, the extent and 

 thickness of the salt bed increasing. This second layer of salt reached a thickness of 

 6.5 U) 7.5 feet, forming, with the lower layer, a bed in the lowest part of the basin 



• For areal Geology see Professional Paper No. 5-5, Ore Deposits of the Silver Peak Quadrangle, J. E. Spurr. 



