1918] 



Davis: The Franciscan Sandstone 



27 



The section shown (fig. 6) is generalized somewhat, and represents 

 the relations at the first locality. The distance from the conglomerate 

 to the Knoxville shales is between 500 and 600 feet. This body of 

 conglomerate lies within a large area of Franciscan rocks. Its rela- 

 tions to the sandstone indicate that it is not an outlier of Knoxville 

 conglomerate preserved in the Franciscan by infolding or by faulting. 

 The conglomerate is seen, in the section exposed by the downcutting 

 of the stream, to be completely enclosed in sandstone of the ordinary 

 Franciscan type, and to be a part of a conformable succession of sedi- 

 ments. The sandstone overlying and underlying the conglomerate is 

 entirely unlike anything seen in the Knoxville around the mountain. 

 The conglomerate passes gradually into the surrounding sandstone by 

 a decrease in the number of pebbles and there are also thin ribs of 

 sandstone intercalated in the conglomerate. The nature of the matrix 

 of the conglomerate is identical with the associated sandstone so that 

 there is no plane of parting between the sandstone and the con- 

 glomerate. 



Fig. 6. Franciscan conglomerate at Mount Diablo. Fs — Franciscan sand- 

 stone; eg — conglomerate; sp — serpentine; Kk — Knoxville shale. 



Conglomerates of this kind, with boulders of glaucophane schist, 

 have been described before. Ransome 42 mentions a conglomerate on 

 Angel Island containing numerous pebbles of a rock bearing blue 

 amphibole. 



Fairbanks 43 reports a boulder of glaucophane schist in a Fran- 

 ciscan conglomerate. The conglomerate consists of a sandstone matrix 

 containing scattered pebbles and a boulder of glaucophane schist two 

 feet in diameter. Fairbanks interprets this to mean one of two things. 

 Either there is a formation of glaucophane schist in the region older 

 than the Franciscan or else there was a period of erosion and redeposi- 

 tion in the Franciscan after the formation of the glaucophane schist. 



Nutter and Barber 44 have suggested that the glaucophane schists 



« Univ. Calif. Publ., Bull. Dept. Geol., vol. 1, p. 197, 1894. 

 ■is San Luis Folio, U. S. Geol. Surv. Folio no. 101, 1904. 



•** Nutter, E. H., and Barber, W. B., On Some Glaucophane and Associated 

 Schists in the Coast Ranges of California, Jour. Geol., vol. 10, p. 738, 1902. 



