University of California. 



I Vol. 2. 



syncline, the middle of which is marked by the depression about 

 Drake's Estero. Along the margins of the syncline the rocks dip 

 steeply, while at its center they are more nearly horizontal. The 

 general axial direction of the fold lies nearly parallel to shore, 

 extending toward the Golden Gate. The development of the fold 

 is doubtless connected with the movements of the block that are 

 discussed in a later section. 



PetrograpJiic Character. — The petrographic character of the 

 shales which form the principal member of the Miocene series 

 has already been the subject of considerable study at other points 

 of the coast. 1 Nothing of great importance has been discovered on 

 the peninsula, not already known from other regions, and their pet- 

 rography would be merely a repetition of what has been said before. 

 In texture they vary from a tolerably granular, sandy phase to 

 what might be called flinty. In Bear Valley and west of Whitten- 

 berg Hill the compact, somewhat vitreous and banded phase is 

 more frequent, though this appears to be an areal rather than a 

 stratigraphical variation. Such portions of the shale are both less 

 porous and less bituminous than the more granular portions. 

 West of Drake's Estero the shales are sandy and the amount of 

 bituminous matter is very much greater than in the more compact 

 portions. This is commonly seen in the fetid character of the 

 water rising from them. 



Origin of the Shales. — The origin of these shales is of an 

 especial interest, since while they are seemingly unique in character 

 there are certain homologies between them and other formations 

 with which they have historically no connection. In thin sections 

 and in fresh hand specimens may frequently be seen, even in the 

 porcelanous varieties, small lath-shaped crystals resembling feld- 

 spar. The ashy or pumice-like character of much of the series at 

 Monterey and Carmelo Bays led Professor Lawson to suggest that 

 possibly volcanic eruptions had been the source of most of the 

 material that composes these shales. The hypothesis finds some 

 support in the chemical composition of the shales, which resembles 



'^Bull. Dept. Geol. Univ. Cal., Vol. i, pp. 1-59; Vol. 2, pp. 1-92; Proc. Cal. 

 Acad. Sci., Vol. 1, No. 1, 3d Series. 



