Vol. 6] Baker: Cenozoic History of the Mohave Desert. 



359 



absence of fossils in the Borate and the fine ashy and shaly tuff 

 members (but one specimen of a Planorbis was found in these 

 beds), as well as the presence of the colemanite, limestone, and 

 gypsum layers, apparently indicates the salinity of the waters. 



There was great volcanic activity before and during the depo- 

 sition of the Rosamond. The larger fragments of lava were 

 most probably derived from flows subject to erosion somewhere 

 in the area tributary to the basin of deposition. Interbedded 

 flows of both acidic and basic lavas make up a part of the Rosa- 

 mond. But the fine volcanic ash was probably blown in by the 

 wind or settled during explosive volcanic outbursts and need 

 not have come from the immediate vicinity. The common view 

 of the origin of calcium borate from solfataras and hot springs 

 associated with the abundant contemporaneous vulcanism is likely 

 to prove to be the correct explanation for the borax beds in this 

 region. 



The sediments of the Rosamond Series were probably laid 

 down mainly as piedmont alluvial debris and as playa deposits, 

 under the same conditions of desert aggradation as operate in 

 the region at the present day, the deposits of the upper Miocene 

 and present periods being indistinguishable in structure, texture, 

 and composition. There were probably times when the climate 

 became humid enough to form at least shallow lakes or ponds 

 of sufficient freshness to permit the existence of gasteropods. 

 Colemanite, gypsum, and limestone were deposited either by hot 

 springs or solfataras in saline lakes, which might have been of 

 shallow depth, or, having been leached from the surrounding 

 rocks, were precipitated during a time or times of evaporation 

 of a former fresh-water lake of considerable depth. Contempor- 

 aneous volcanoes added breccias and lava flows, as well as fine 

 ash and coarser pyroclastic materials to the mass of Rosamond 

 sediments. 



The alternative hypothesis is that the Rosamond was depos- 

 ited for the most part under climatic conditions of relative 

 humidity, but so rapidly and with the source of the sediments 

 so near that there was not time enough for chemical decompo- 

 sition to take place and the materials were moved such a short 

 distance that the amount of mechanical wear was negligible. 



