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University of California Publications. [Geology 



of granitic materials and lavas and matrices of finer arkoses, 

 volcanic ash, and chemical precipitates. Tuff -breccias, finer tuffs, 

 and shales and mudstones with interbedded layers of gypsum, 

 calcium carbonate, and borax minerals occur in subordinate 

 amounts. Conglomerates of water-worn boulders are of rather 

 rare occurrence. 



EOSAMOND EXPOSUEE NOETH OF BAESTOW 



The Rosamond here attains its maximum known development 

 both in thickness and diversity. The approximate thickness of 

 the series cannot be much less than a mile, although faulting 

 has rendered difficult the determination of the exact thickness. 

 In this exposure there are at least five mapable units, but not 

 all of the five have yet been found in any other single locality. 



Basal Breccia Member. — The base of the Rosamond is every- 

 where marked by an erosion unconformity with a basal breccia 

 resting on weathered surfaces of granitic rocks and lavas (pi. 

 35a). At the base of the south limb of the Barstow syncline a 

 distinct basal breccia of mapable thickness occurs, resting on 

 the eroded surfaces of both granodiorite and a basic andesite 

 or acid basalt, and containing fragments of both of these rocks. 

 The member is mainly dark brick-red in color and at least sev- 

 eral hundred feet thick, although the displacements caused by 

 two strike faults makes its exact thickness uncertain. The frag- 

 ments are mostly angular or subangular, but some rounded 

 pebbles and boulders which probably owe their form to exfoli- 

 ation, as indicated by their rough surfaces, are found. The 

 fragments are rather small, ranging up to six inches in diameter. 

 The matrix is mainly arkosic, being largely disaggregated frag- 

 ments of quartz and feldspar derived from the granodiorite. 

 A basal breccia of this same type was found resting on granitic 

 rock at the base of the north limb of the syncline, at the north- 

 west end of the exposure, but farther east the basal beds contain 

 such a large proportion of volcanic ash that they may more 

 appropriately be included in the next member. 



Tuff-breccia Member. — (PI. 35b.) This differs from the basal 

 breccia member in being mainly composed of finer fragments, 

 in generally containing less granitic material, and in having in 



