1922] 



Vaughan: Geology of San Bernardino Mountains 



361 



In .the section exposed in Arrastre Creek above the limestone this 

 formation has suffered much the same sort of disturbance as the 

 Arrastre quartzite in lower Arrastre. Some has been metamorphosed 

 so as to greatly resemble the associated granitic gneiss and the whole 

 mass is cut by pegmatite dikes. Along the south side of Sugarloaf 

 for about 1500 feet from the top of the ridge the quartzite retains its 

 true character, but below this there is a more or less continuous zone 

 where the quartzite has been highly altered by the intruding granite, 

 as in Arrastre Creek. 



A thin section was made of a piece of pinkish grey sacchroidal 

 quartzite from west of Round Valley. The quartz grains are rather 

 intimately intergrown, possibly due to secondary deposition of silica, 

 but the original outlines of the fragments cannot be recognized. Most 

 of the quartz exhibits undulatory extinction. A few grains of mag- 

 netite and titanite are present, and also flakes of muscovite and biotite. 

 While the quartzite is so variable that even a large number of sections 

 would not be wholly adequate to describe it, this specimen is typical 

 of by far the larger part of the formation. 



Age op the Older Sediments 



No fossils were found in the limestone or quartzites and any state- 

 ments as to their age must be based entirely on their relations to the 

 granites and their lithological resemblances to other rocks of known 

 age. They are cut by granites belonging to two periods of intrusion, 

 one of which is probably late Jurassic. The age of the other is 

 unknown. These relations, however, alone are sufficient to suggest 

 that the sediments are of ancient date, probably Paleozoic. 



Hershey 8 has described a quartzite-limestone series at Oro Grande 

 and because of its lithologic resemblance to the lower Cambrian series 

 of Inyo County described by Walcott''' has classed it as being of the 

 same age. This series is by no means so thick as that in the San 

 Bernardino Mountains, for Hershey says that the thickness of the 

 limestone is "probably no more than 200 feet" as compared to the 

 4500 feet of Furnace limestone. Darton 10 describes a series of Cam- 

 brian and Carboniferous limestones, quartzites, and shales in the Iron 

 Mountains north of Cadiz, about 60 miles east of the northeast corner 



8 Hershey, C. H., Some crystalline rocks of Southern California, Am. Geol., 

 vol. 29 (1902). 



Walcott, Chas. D., Lower Cambrian rocks in eastern California, Am. Jour. 

 ScL, vol. 49 (1895), p. 141. 



io Darton, N. H., IT. S. G. S., Bull. 613, Part C, Santa Fe Eoute, 1915. 



