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University of California Publications. [Geology 



The greater part of the sediments — the granitic and volcanic 

 breccias — had their origin in nearby areas tributary to the place 

 of deposition. These areas must have possessed considerable 

 elevation, in order to produce the great bulk of sediments. The 

 lack of mature chemical weathering, shown by the arkosic ma- 

 terial, and of any considerable mechanical attrition, because 

 the boulders or smaller fragments are angular or subangular in 

 outline, implies rapidity of erosion and deposition. For the 

 above reasons it is thought that the coarser part of the series 

 was derived from recently uplifted areas of considerable eleva- 

 tion. This seems to require an epoch of mountain-making some- 

 time between the middle of the Miocene and the beginning of 

 the Pliocene periods. 



There is a very conspicuous and almost total absence of the 

 decomposed products of mature weathering. It is probable that 

 even the finer clays and mudstones of the Borate and of the fine 

 ashy and shaly tuff members are mainly composed of fine vol- 

 canic ash. The absence of products of rock decomposition goes 

 far towards proving that the Rosamond was deposited in an arid 

 climate similar to that of the Mohave Desert todaj^. But this 

 analogy should not be pushed too far, for the interbedded layers 

 of colemanite, gypsum, and limestone were most probably depos- 

 ited on the evaporation of a body of water of considerable depth, 

 since the colemanite layer is from five to thirty feet in thickness, 

 layers of pure gypsum several inches thick are found, as well 

 as more considerable thicknesses of what is probably chemically 

 deposited limestone. An alternative hypothesis, that these min- 

 erals had their immediate origin in hot springs and solfataras 

 opening directly into shallow lakes, perhaps only of seasonal 

 duration, or in playas, has much to commend it, especially when 

 considered in connection with the numerous evidences of shallow 

 water deposition. These evidences comprise ripple marks, sun 

 cracks, and rain prints, which are found on the finer as well as 

 the coarser beds, and the layers of finer, breccia and conglom- 

 erates interbedded with the fine shales and tuffs. Shallow lakes 

 or ponds probably existed at times during the deposition of the 

 fossiliferous tuff member, for they seem to be necessary to ac- 

 count for the presence of the gasteropods. The paucity or 



