SOUTH AND WEST OF MOUNT DIABLO 291 



Tejoii, but belong to the Megaiios period of deposition; tliat neither are 

 to be correlated with the typical Tejon of the type section. The first true 

 Tejon in this section is represented by the beds of Dickerson's Balano- 

 phyllia zone. In this section as well as on the north side of the mountain 

 these latter beds, the true Tejon, are found unconformable on top of the 

 Meganos. 



Near the western edge of the Mount Diablo quadrangle, beds belonging 

 to the Meganos Group are found unconformably beneath beds containing 

 the "Balanophyllia fauna." These outcrops are found on the north side 

 of the ridge north of Pine Canyon, extending to the west onto the Con- 

 cord quadrangle. Not as much detailed work has been done by the writer 

 on the Meganos portion of the section in this area to the west of Mount 

 Diablo as has been done on the section in the area to the north of the 

 mountain ; for this reason only a very general statement as to the lithology 

 and fauna of this section can be made. 



STRATIGRAPHY AND LITHOLOGY 



The basal conglomerate of the Meganos in this western area is found 

 on the ridge to the south and west of the mouth of the Arroya del Cerra. 

 Here the conglomerate rests on dark shales and carbonaceous brown sand- 

 stones, which very probably are of Cretaceous age. The conglomerate 

 has a maximum thickness of close to 20 feet. The lower portion of the 

 conglomerate is fairly coarse, the pebbles being composed largely of shale, 

 limestone, sandstone, and conglomeratic sandstone, together with some 

 quartzitic and igneous boulders. Some of the boulders of sandstone are 

 angular and are as much as a foot to a foot and a half in length. Typical 

 Chico fossils were found in some of the conglomeratic sandstone boulders. 

 The pebbles in the upper l^eds of this conglomerate consist chiefly of fine 

 angular fragments of shale, derived apparently from the dark shales im- 

 mediately below. 



Above the basal conglomerate, just described, is, roughly estimated, 

 150 to 200 feet of medium fine, yellow-brown, fossiliferous sandstone, 

 and above this about 1,800 feet of dark-colored shales, fine sandstone, and 

 shaly sandstone. 



The line of division between the Meganos and Tejon is marked by a 

 narrow band, possibly two feet thick, of fine conglomerate. The pebbles 

 at the base consist of quartz, black chert and red chert similar to the 

 clierts of the Franciscan, and shale. Above this the pebbles consist almost 

 entirely of angular fragments of shale in a matrix of coarse, light gray 

 sandstone. Thus here again we apparently have a true basal conglom- 

 erate. 



