l6o CALIFORNIA ACADEMY OF SCIENCES [Proc. 30 Ser. 



The geologic and topographic features of the series are 

 the same as everywhere in the coast ranges both north and 

 south of the Bay of San Francisco. In this field the series 

 is confined in its occurrence to the axis of the chief range 

 extending west of the Coalinga district, or, more accurately, 

 to a few prominent areas within that range. 



There are two or three principal areas of Franciscan and 

 serpentine rocks, separated to a considerable extent by an 

 area of Cretaceous strata. One of these lies to the south of 

 the upper tributaries of Alcalde Creek (or Warthan Canyon), 

 and extends from there southeasterly to Cottonwood Pass ; 

 another extends from the upper branches of Los Gatos Creek 

 northward toward the Panoche Valley and the tributaries of 

 the San Benito River, and therefore includes the New Idria 

 quicksilver district and the San Carlos and San Benito peaks. 



The most extensive formation in this area is undoubtedly 

 serpentine. To the south and west of New Idria, serpentine 

 is almost the only rock to be seen for many miles. The 

 sedimentar}' rocks of the Franciscan series are mostly con- 

 fined to the southern and western borders of the area. 



Cretaceous Strata. 



Lying along the eastern margins of the Franciscan areas 

 and filling wide spaces between, are Cretaceous rocks, form- 

 ing a stratified series of great thickness and dipping steeply 

 toward the Great Valley. An important area of Cretaceous 

 rocks is that between the Alcalde and Los Gatos creeks near 

 Coalinga. 



The Cretaceous strata include both the Knoxville and 

 Chico divisions, with the intervening Horsetown Beds appar- 

 ently omitted. The usual nonconformity between these mem- 

 bers has not been proved in this field directly, though there 

 are abundant grounds for believing that it exists. 



The Knoxville consists of a thick series of dark clay shales 

 and thin-bedded sandstones, lying next to the Franciscan 

 rocks. They have been particularly noted along the head 

 waters of Alcalde Creek, near the Fresno Hot Springs, on 



