24 THE SALTON SEA. 



into the rain-washed alluvium of the basin slopes that the boundary is indeterminate and 

 the mutual relations undecipherable. 



Superposed on the major features of the basin, as thus described, are several minor 

 features which require brief notice. First is the line of "mud volcanoes," evidently decayed 

 fumaroles, which approximately parallels the axis of the trough. These are well developed 

 near Volcano Lake, and are now represented by several hot springs, boiling pools, etc. 

 A former more intense activity is indicated by groups of inactive mud cones, extensive 

 deposits of sulphur and alum, and similar evidences of fumarole action. Another group 

 of hot springs and mud geysers was located just north of the buttes which are now islands 

 in the Salton Sea. These were described by several early explorers and especially by Le 

 Conte.' They were considerably more active than the Volcano Lake group, but not so 

 extensive. They are now under water and their present condition is not determinable. 

 Some 6 miles north of these, near the railway station of Lano, is another group, somewhat 

 smaller and much more nearly extinct. Activity here is limited to a very small flow of 

 muddy water and the escape of an occasional bubble of gas. 



The buttes above mentioned as being present islands in the Salton Sea are exclusively 

 of volcanic rocks, including pumice, glassy obsidian, and a variable volcanic tuff containing 

 many fragments of the pumice and obsidian. No crater or other vent is visible, and the 

 buttes are probably simply the summits of a larger mass of eruptives, probably of Ter- 

 tiary age and disturbed and buried by the post-Tertiary movement and alluviation. Other 

 small bodies of effusives and tuffs are reported by Mendenhall as interstratified with the 

 lower members of the Mud Hill Series in the Carrizo Creek Valley. An olivine basalt, 

 apparently effusive, was found by J. C. Jones and the writer on the summit of the Peninsula 

 Range near Jacumba. So far as known, these are the only representatives within the 

 Cahuilla Basin of the Tertiary volcanism so marked farther north. 



Among the more recent deposits mention should be made of two areas of sand dunes, 

 both of which are still in course of formation and change. The larger of these is the com- 

 plex of "Los Algodones," in the northeastern part of the basin, as noted on Mr. Sykes's 

 map. It is a single great dune area of usual character and offers little of interest. The 

 second dune area is in the embayment between the Santa Rosa Mountains and Superstition 

 Mountain and lies just south of the end of the former range. It is remarkable chiefly 

 because the dunes are few enough and far enough apart to permit the development of the 

 crescentic form characteristic of a freely moving dune. One of the crescentic forms is 

 shown in Plate 6 a and b. Many others are scattered over the surrounding slope, making the 

 only good example of these forms known in the United States and probably one of the best 

 in the world. 



Of the minor features of the basin the only additional one which needs notice is the 

 ancient beach-line which surrounds the basin about 40 feet above sea-level and which is 

 described by Mr. Sykes. This is well marked in almost all quarters of the basin. On the allu- 

 vial slopes it appears as a deep wave-cut terrace, as illustrated in Plate 6 c and d. On rock 

 walls and spurs it is evidenced mainly by the crusts of travertine, described elsewhere in 

 this volume by Professor Jones. Below this highest beach-line, wherever the topography 

 has permitted their preservation, there appear numerous minor strands, each a few feet 

 or inches below the next, and the series extending downward to the terrace corresponding 

 to the high-water level of the Salton Sea. This series of ancient strands is well illustrated 

 by Plate 29 b, where the influence of the gravel accumulations along the terrace lines has so 

 controlled the vegetation as to mark the strands very clearly. It is difficult to regard this 

 series of strands as anything but the record of a lake which once occupied the basin and 

 which disappeared by evaporation as the present Salton Sea is disappearing. This was the 

 interpretation made by the late Professor Blake, and the ancient lake has been named 



' LeConte, Amer. Jour. Science, 1855. 



