Vol. 4| Osmont. — Geological Scrtio)i of Coast Ranges. 



While the richest ore is usually at the contact between the opal- 

 ized zones of serpentine and the sandstone or shale country rock, 

 there are sometimes large bodies of low grade ore directly within 

 the opalized area and many feet away from the contact. One 

 instance* is known where the ore body consists of radiolarian 

 chert, along a contact with serpentine, which has been filled with 

 veinlets of opal carrying cinnabar. 



Serpentine is very abundant throughout all the above men- 

 tioned areas, but it was not found practicable to map it. In some 

 cases, especially near Knoxville, the masses of serpentine are 

 more than a mile in width, their great size seeming to preclude 

 the idea of their being dikes and to suggest that they must repre- 

 sent intrusive sills or laccolites. 



SHASTA-CHICO. 



The eastern portion of both sections is largely made up of 

 Cretaceous shales and sandstones, most if not all of which prob- 

 ably belong to the lower Cretaceous or Knoxville series. f 



Lithological Character. — This series consists of an enormous 

 thickness of rather hard tawny yellowish sandstone, interbedded 

 in monotonous succession with dark blue fissile shales, with occa- 

 sional thin beds of dark blue limestone. The sandstone strata 

 are usually less than two feet thick, and almost never more than 

 ten, while their regular alternation with soft shale made the bed- 

 ding very distinct and characteristic. 



Stratigraphy. — While standing at high angles, these beds do 

 not show any important faulting along either of the sections. 

 Prom near Knoxville, where they appear to overlie a large 

 laccolite of serpentine, they extend in unbroken succession 

 with steep northeasterly dip, to the head of Capay Valley, at 

 Kumsey, where they are covered by Tertiary gravels and sand- 

 stones. The average angle of dip from Knoxville to Rumsey 

 cannot be less than 45°, which would give the series a thickness 

 of four miles, and this does not represent the whole of the accumu- 

 lation of sedimentary beds, since the upper limit is not exposed 



*Notecl by Lawson at the Great Western Quicksilver Mine, Napa County, 

 and communicated verbally to the writer. 



tBecker, Mon. XIII, U. S. G. S. Quicksilver Deposits of the Pacific- 

 Slope. 



