36 University of California Publications in Geology [ VoL 11 



The shale flakes of the Franciscan are like the "clay galls" of 

 Grabau. No other origin has ever been suggested for these other than 

 that outlined in the description of the Spokane formation. Accord- 

 ing to Grabau, 57 they are very common in sandstones known certainly 

 to be subaerial in origin, and their presence in a sandstone may be 

 regarded as practically positive evidence of a subaerial origin. 



Occasional small shale fragments occur in some of the Knoxville 

 sandstones at Mount Diablo. The Chico sandstones used in the build- 

 ings at Stanford University also contain small fragments of shale. As 

 far as the writer is aware, no marine fossils occur in the sandstones at 

 the points where these shale fragments occur and they probably mark 

 a subaerial facies of sedimentation. However, their presence in forma- 

 tions which frequently contain marine fossils, indicates the need for 

 caution in the use of shale fragments as a criterion of continental 

 origin. 



LENSES OF CONGLOMERATE 



The numerous lenses of conglomerate, at various horizons through- 

 out the sandstone, favor the fluviatile hypothesis rather than the 

 hypothesis of marine deposition. 



Beach conglomerates usually show rounded boulders. They are 

 generally well washed and the matrix is not normally feldspathic. 

 Marine conglomerates should be more persistent than these conglomer- 

 ates are. The frequent repetition of horizons of conglomerate is in 

 favor of the fluviatile hypothesis. The lens-like character of the con- 

 glomerates at once leads to the idea that they represent sections of 

 old stream courses. On the fluviatile hypothesis these conglomerates 

 represent pebbles accumulated in stream channels which were aban- 

 doned by the aggrading streams from time to time. The great variety 

 of lithologic types represented in the conglomerate is in accordance 

 with this view of the nature of the conglomerates. 



On the assumption of continental deposition of the sandstone, one 

 can easily explain the presence of numerous boulders of Franciscan 

 rocks in these conglomerates. They represent portions of the forma- 

 tion deposited at an earlier date, consolidated, and subjected to local 

 erosion with deposition elsewhere in the accumulating sediments. 



•"■"Principles of Stratigraphy, pp. 564, 711. 



