1918] Davis: The Franciscan Sandstone 35 



Deposits of the sort suggested are being formed, under conditions 

 somewhat similar to those postulated, on the eastern shore of the 

 Gulf of Lower California, on the west coast of the state of Sonora. 

 They have been described by W J McGee/' 5 



The region is one of broad plains and rugged mountains, the rocks 

 of which consist mostly of granites and old schists. In the mountains, 

 the rocks are nearly bare and the plains are covered with a thin 

 veneer of coarse alluvium. The precipitation is very slight. Rock 

 decomposition is practically absent and the mechanical debris is 

 washed down by occasional sheet floods. The sands accumulating on 

 the outer margin of the wave-cut terrace and in the reentrants of the 

 shore are typical arkose. They consist of rather coarse angular frag- 

 ments of quartz, feldspar, and mica, which are usually clean but 

 sometimes mixed with more finely broken material. Such a deposit on 

 cementation would result in a rock much like the Franciscan sand- 

 stone. 



SHALE FLAKES 



One feature of the Franciscan sandstone is the occurrence in it of 



flakes of shale, often with angular outlines, reaching an inch or so in 



cross section and about one-sixteenth of an inch thick. Locally these 



may be very numerous. These appear to be similar to certain mud 



flakes in the Spokane formation, described by Calkins : 56 



A prominent characteristic of the sandstone of the Spokane formation is the 

 presence in many layers of abundant flat masses of dark argillite from a small 

 fraction of an inch to two inches in diameter, similar to the finest-grained portion 

 of the shales. Commonly these masses, whose larger dimensions are in the bed- 

 ding planes, are smooth and well rounded, like pebbles. The rock containing them 

 differs in two respects from typical conglomerate, however; first, the volume of 

 the matrix of sandstone greatly exceeds that of the pebbles, and, second, the peb- 

 bles are virtually homogenous in composition and consist of a kind of material 

 abundantly interbedded with the sandstone. These beds grade into others in 

 which the argillaceous particles are sharply angular. It is believed, in view of the 

 common association of these rocks with mud cracks, that the argillaceous frag- 

 ments were derived from mud flakes curled up and loosened in the process of dry- 

 ing, and buried by the next deposit of sand. The rounding of the fragments in 

 certain beds is certainly the result of attrition, but as fragments of mud could 

 hardly survive transportation by water for any considerable distance it is prob- 

 able that some of them were rolled by the wind after drying. 



5 "> McGee, W J, The Formation of Arkose (abstract), Science, n. s., vol. 4, 

 p. 962, 1896. 



•™ Calkins, F. C, Geology and Ore Deposits of the Phillipsburg Quadrangle, 

 Montana, U. S. Geol. Surv., Professional paper no. 78, p. 46, 1913. 



