22 THE SALTON SEA. 



Petrologically, the available information is similarly meager of detail. On the north 

 and northeast the Chocolate, Cottonwood, San Bernardino, and San Gabriel Mountains 

 form one range, nearly continuous from the Colorado River, until they merge with the 

 Sierra in the highlands of Tehachapi. Where this range has been examined it is composed 

 almost entirely of granites, gneisses, and ancient schists. From his studies in this range 

 and to the north, Harder distinguishes two general divisions of the granitic rocks; the first 

 is probably Pre-Cambrian and associated with gneisses and schists; the second is a later 

 intrusive, probably Mesozoic. Overlying the early granitics, but antedating the intru- 

 sives, are fragments of slates, quartzites, and limestones of unknown age, and usually much 

 metamorphosed and disturbed. The studies of Mendenhall 1 indicate a similar character 

 for the Santa Rosa Mountains, Fish Creek Mountain, and other outliers of the Peninsula 

 Range which border the basin on the southwest. 



Along the basin-ward foot of the San Bernardino Range, throughout nearly its whole 

 extent, are irregular and much eroded hills of poorly consolidated conglomerates, sand- 

 stones, and shales. In most cases these strata have been greatly disturbed and their 

 original relations are not clearly decipherable. It seems probable, however, that they 

 consist of three fairly well-defined members: (1) a basal conglomerate resting normally upon 

 an eroded surface of schists and granites, such as make up the core of the range; (2) lying 

 conformably upon this, a thick member of coarse, arkose sandstone, usually reddish in 

 color and of quite uniform texture; (3) an upper member of very variable sandstones and 

 clays, mostly thinly bedded and showing many probable minor unconformities. Aside 

 from the vertical variation as thus noted it seems also that the fineness of the material 

 in each of the main divisions increases as one recedes horizontally from the mountains. 

 Thus the lower conglomerate decreases in thickness and its constituent pebbles become 

 smaller, the middle arkose member shades off into a sandstone of much finer and more 

 uniform grain, and the top member, while almost entirely of sandstones at the foot of the 

 mountains, becomes almost entirely of clays as one approaches the basin floor. In many 

 places these clays contain thin crusts of gypsum and in one place, about 5 miles north of 

 the railway station of Pope, they contain an interbedded stratum of mirabilite or hydrous 

 sodium sulphate, several feet in thickness. This series of strata will be designated the 

 Mud Hill Series, from the Mecca Mud Hills, where it is perhaps best developed. At that 

 locality the section corresponds to perhaps 3,000 vertical feet, 2 but it is impossible to be 

 sure of this or to draw a general section because of the very broken character of the strata 

 as exposed and the impossibility of determining whether observed differences in the visible 

 fragments of the strata indicate strata of different vertical position in the section or simply 

 variations in the same stratum at different distances from the mountains. 



Beyond the southeastern end of the Santa Rosa Mountains similar sandstone and 

 clay strata surround the granite core of Superstition Mountain and form several series 

 of low hills which flank the outliers of the Peninsula Range much as the San Bernardino 

 Range is flanked by the strata just described. It is scarcely possible definitely to correlate 

 these strata with the similar series on the northeastern side of the basin, but the lithologic 

 similarity is apparently complete and it seems very probable that both exposures belong 

 to the same series and to the same genetic condition, whether or not they be exactly syn- 

 chronous. Provisionally, all will be regarded as belonging to the Mud Hill Series. 



No fossils have been found in the Mud Hill Series at the type locality at Mecca or, 

 indeed, anywhere on the northeast side of the basin. In the valley of Carrizo Creek, however, 

 Blake discovered strata of partially consolidated clay and sand containing oysters, pectens, 

 and other marine shells of Miocene age. According to Mendenhall these fossiliferous beds 

 are here the basal member of the sedimentary series. This, in connection with its general 



1 hoc. cit., pp. 10-14. 2 Mendenhall estimates 4,000 to 5,200 feet, he. cit., p. 12. 



