Vol. 4] Osmont. — Geological Section of Coast Ranges. 



83 



mergence is going on at the present time in the vicinity of San 

 Francisco Bay. In recent excavations made at Shell Mound 

 Park, between Berkeley and Oakland, it was found that the base 

 of the shell beds are now four feet below the ordinary high-tide 

 mark. Since it is evident that the mound is a "kitchen-midden," 

 and therefore built on the land, this fact proves that there has 

 been in very recent times a subsidence of at least four feet. 



GeomorpMc Cycle. — The topography of this section then may 

 be placed at a somewhat advanced stage in the geomorphic cycle, 

 and, since this cycle must have been inaugurated not earlier than 

 the beginning of the Pleistocene, the observer is immediately 

 impressed with the enormous amount of erosion which has taken 

 place, and the vast space of time represented by this the most 

 recent period of geological history. 



HISTORICAL RESUME. 

 After the intrusion of the pre-Franciscan strata by the 

 granitics, a great period of erosion occurred, as shown by the 

 great unconformity existing between Franciscan and the older 

 rocks. 



Upon this old, well worn surface the Franciscan series was 

 laid down, the variety of the sediments giving evidence of fre- 

 quent oscillations during their deposition, while the sharp fold- 

 ing and faulting that has taken place, and the volcanic intrusions, 

 attest the immense amount of movement subsequent to their 

 deposition. 



During Shasta-Chjco times probably the whole of this area 

 was deep under the sea and the Sierras were undergoing ei'osion, 

 for lying unconformably upon the Franciscan, as well shown 

 in many places outside of this field, and suggested by the heavy 

 chert conglomerate of Capay Valley, is a vast accumulation of 

 thin-bedded shales and sandstones in monotonous rhymical suc- 

 cession, indicating deep-water deposition under certain peculiar, 

 and as yet unexplained, conditions. At the most conservative 

 estimate the Shasta-Chieo strata in this territory have a thick- 

 ness of five miles, and they are probably considerably thicker 

 than this. The writer has no palaeontological evidence as to 

 whether or not they include the Chico. Certainly no heavy beds 



