STRATIGRAPHY 567 



along the mountain front north of Santa Cruz gave details of its char- 

 acter and relation to the Bermejo. 



The Cajones limestone comprises a series of intercalated sandstones, 

 claystones, and limestone conglomerates that may have an aggregate 

 thickness of 800 feet. Most of this thickness is occupied by limestone 

 conglomerate. The pebbles are red shale, dark quartzite, red orthoclase 

 granite, much vein quartz (which is commonly white, but may be black, 

 brown, or rosy), and a small percentage of other rocks. The bedding 

 is quite thin, few individual beds exceeding 20 feet in thickness (see 

 figure 7). 



This iormation is thought to be the result of an invasion of the sea 

 over the eroded, pebble-littered surface of the Bermejo formation. 



SAN YSIDRO FORMATION 



The San Ysidro formation was named by the writers from the settle- 

 ment of San Ysidro, on the Santa Cruz Trail, about 75 miles west and 

 30 miles south of Santa Cruz. The westernmost exposure noted is about 

 14 miles west of San Ysidro, and the formation occupies the surface 

 from that point to about eight miles east of the town, where it covers 

 the lower flanks of a steep ridge that is capped by rocks of the Materal 

 formation. 



The San Ysidro formation is believed to be Tertiary, although no 

 paleontological evidence to support such a belief was found. The lith- 

 ology and "new" appearance of the beds, their very unconformable rela- 

 tion to the underlying Material, and their absence between any two of the 

 other formations described, suggesting that they post-date the Cajones 

 limestone, which would make them at least as late as Cretaceous, are the 

 bases for the classification. 



The thickness of the formation is thought to be about 1,000 feet. 

 It may be half that, or as much as 5,000 feet. It comprises shales, a 

 few thin sandstones, and at least one bed that is composed chiefly of 

 volcanic ash. The general color is light gray to white, but the thin sand- 

 stones lend contrast, as they are, almost without exception, dark red. 

 Individual beds of shale are thin, and exposures near the trail showed 

 neither fissility nor lamination. 



The tuffaceous bed mentioned is whitish gray on both weathered and 

 fresh surfaces. The bed would be almost pure white were it not for an 

 admixture of tiny flakes of biotite which gives a "pepper-and-salt" appear- 

 ance to a closely examined surface. Distinct bedding and imperfect 

 ripple-marks indicate that the material was water-laid. 



