52 



BULLETi:iSr 61, U. S. DEPARTMENT OF AGKICULTUEE. 



zation. From IS to 35 feet a sulphate period followed, and this in turn was closed by 

 the chloride period. Two periods of desiccation are indicated in the closing stages — 

 the present one and one (recent geologically) at some unknown time before. 



The marginal bore alsa indicates an interruption in the desiccation. A passage 

 from the trona to the sulphate period was followed by a trona period. The last trona 

 period passes gradually into the chloride period, with possible indications of another 

 interruption at 25 to 30 feet depth. More or less sedimentation marked this portion 

 of the deposit. The final capping with a mud layer 13 feet thick closed the cycle of 

 events at this point. 



The important question of what became of the residual mother liquor which must 

 have covered the saline bed at the close of the last desiccation period has not been 

 discussed. The suggestion by J. Walther, quoted by Clarke,^ that residual bitterns 

 might be absorbed by wind-blown sands, and by capillarity brought to the surface, 

 wind eroded, and carried awa\', occurs as a plausible explanation. Undoubtedly 

 some such action took place locally, but it could not have been on a sufficient scale to 

 account for the removal of all of the mother liquors. The fact that the upper portion 

 of the central bed contains a large proportion of sodium chloride and a brine lower 

 in potassium content than the brine beneath suggests that the closing stages of desic- 

 cation must have closely paralleled present conditions in Death Valley. Searles Lake, 

 in passing through the last stages of desiccation, must have deposited sodium chloride, 

 as well as other salts, over a much larger area than that occupied by the present cen- 

 tral bed. The shallow lake of mother liquor occupying the central depression must 

 have received periodic accessions of saline material from these marginal deposits. 

 Continued over a long time the effect would be to build up a bed of saline material 

 in which the content of potassium salts would not be conspicuous and which would 

 contain the diluted original mother liquor absorbed in its interstices. Continual 

 accession of salines from the margins would result in a top bed of saline material 

 comparatively poor in potassium salts. This explanation appears to me to be the 

 mo.st reasonable. 



The central salt bed over practically the whole area of 12 square miles contains in 

 its interstices a brine which, below the top bed of sodium chloride, is characterized 

 by a relatively high content of potassium. According to Dolbear, the brine con- 

 taining the high content of potassium salts is confined to a vertical horizon of some 

 47 feet. Below this horizon the brines contain relatively less potassium salts. The 

 following is an analysis of the rich brine taken from bore hole A7 on the N-S center 

 line and just south of the center of the salt area: 



Analysis of brine from Searles Lake, expressed in percentages of the anhydrous residue. 

 [Simple coU33tel by E. E. Fres; aiilysis by W. H. Ro33, of tha Barsii of SDih.] 



Constituent. ' 



Per cent. 



1 

 Constituent. ' Per cent. 



Constituent. 



Per cent. 



Na 



33.57 

 6.06 

 .01 

 None. 

 None. 

 None. 

 Trace. 



Mn None. 



Cu 1 None. 



AljOj i .012 



FejOj , .003 



' SiOs 023 



CI 37.02 



Br 094 



I 



0.004 



K 



SO4 



12.96 



Li 



CO3 



6.70 



Rb 



PO4 



.30 



Ca 



NO3 



None. 



NHi 



AsOj 



.083 



Mg 



B4O7 



3.00 









This is of the nature of a residual mother liquor. It consists of chlorides in greatest 

 amount, sulphates, carbonates, practically no bicarbonates, and borates. It is con- 

 spicuous by the presence of bromine, iodine, and arsenious oxide. The sodium- 

 potassium ratio in this brine is 5.5. The average of 14 of the samples from the deep 

 boring, omitting results upon crystals and Nos. 217 and 215, is 15.5. This would indi- 

 cate tnat the water collected in the early stages of Searles was not unlike that of the 

 present lakes in which the sodium-potassium ratio is 20. 



Supplementary analyses by A. R. Merz upon samples collected by E. E. Free are 

 given in Table XXIX (Appendix). 



The brine body is contained in a mass of coarsely crystalline material, more or less 

 honeycombed. The portion occupying the central mass of salt is richest in potassium 

 ealts and borates below the upper salt crust of 18 feet thickness and above the 65- 

 foot level. 



Bui. No. 491, U. S. Geol. Survey, p. 224. 



