THE TUFA DEPOSITS OF THE SALTON SINK. 



By J. Claude Jones. 



One of the most characteristic deposits left by the Quaternary lakes of the Great 

 Basin are the masses of calcareous tufa found in their former basins. Those of ancient 

 Lakes Bonneville and Lahontan especially have been noted by the geologists who examined 

 them and they have been thoroughly studied by Gilbert 1 and Russell. 2 In accounting 

 for these deposits it was supposed that they were due to chemical precipitation resulting 

 from the concentration of the lake waters by evaporation or through the mechanical 

 expulsion of loosely combined carbon dioxide through wave-action. 



An examination of specimens of the tufa at present forming in the shrinking Salton 

 Sea disclosed the intimate association of vegetation with the deposit and suggested an 

 alternate hypothesis as to its origin. The present study, made possible through the kind- 

 ness of Dr. D. T. MacDougal and Mr. E. E. Free, has indicated that this hypothesis 

 applies to the older tufa deposited in the same basin by the former Blake Sea, and it is 

 not impossible that much of the tufas of the other Quaternary lakes have a similar origin. 



THE PRESENT TUFA DEPOSITS. 



During 1906 and 1907 the rising water of the present Salton Sea killed the shrubs 

 and bushes growing on its former shores. As the water receded the dead vegetation 

 remained standing and in 1910 it was noted that a calcareous incrustation or tufa was 

 forming on the submerged twigs and branches. When this tufa is first taken from the 

 water it is soft, gelatinous, and dark greenish-gray in color. On drying it hardens, becomes 

 much lighter in color, and takes on the characteristic appearance of a tufa. 



The deposit is a millimeter or so in thickness, with a roughened surface covered with 

 minute warty excrescences. It is not a simple uniform coating, but is made up of a multi- 

 tude of protuberant individual deposits. These vary in size from a pin-head up to 0.5 

 centimeter in diameter and, broadening in their outward growth, coalesce, forming a fairly 

 uniform coating with interspaces scattered here and there where little or no deposit has 

 formed. 



A hand-lens disclosed many thread-like green algae originally projecting beyond the 

 mass of the tufa but matted down over its surface with the drying out of the material. 

 By dissolving the tufa with weak acetic acid the algae were isolated. This material was 

 referred to Mr. C. L. Brown, of the University of Nevada, who identified the algae as a 

 species of Calothrix, probably either C. thcrmalis or parietina. 



In thin sections of the tufa the carbonates were found to be very finely crystalline 

 and with a characteristic cloudy appearance. While it was possible to recognize individual 

 crystals under the higher powers of a compound microscope, yet the cloudier material 

 could not be resolved into separate grains. Under an oil immersion objective the cloudy 

 appearance disappeared almost entirely and it is possible that it was caused by the reflec- 

 tion of light from the surfaces of submicroscopic crystals. 



1 G. K. Gilbert, U. S. Geol. Surv. Monographs, vol. i, 1890. -' I. C. Russell, U. S. Geol. Surv. Monographs, vol. n, 18S5. 



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