14 University of California. [Vol. 2. 



With regard to the flinty beds which reach 100 feet or more in 

 thickness in some places, and in others are scattered in thin bands 

 through the somewhat chalky strata, it must be admitted that the 

 presence of the circular areas, undoubtedly of organic origin, 

 favor the view that the beds are almost entirely of such origin, the 

 solvent power of the sea water during and after the slow formation 

 reducing the silicious tests to a structureless mass. There seems 

 to be considerable evidence that the less flinty strata containing 

 alumina and alkalies are in part of volcanic origin, but it does not 

 seem to the author possible that the bulk of these rocks was thus 

 formed. At Point Sal, in the lower Miocene, several thousand feet 

 below the bituminous shales, there are three strata made up wholly 

 of volcanic ash. The volcanic character is most strongly pro- 

 nounced, and it would seem reasonable that if volcanic ash played 

 an important part in the formation of the silicious rocks, which are 

 considerably younger, it would somewhere be clearly recog- 

 nizable. The great thickness of the bituminous shales with a fairly 

 constant character would demand an exceedingly prolonged and 

 continuous eruption of great magnitude, of which we have discov- 

 ered no other record. In the San Antonio Valley, in Monterey 

 County, there is quite an abrupt transition from a coarse arkose 

 sandstone below to the bituminous shales. The strata are conform- 

 able, and it would appear that the beginning of the period of depo- 

 sition of the shales must have been coincident with a depression 

 which probably removed the shore line to a considerable distance, 

 as the fragmental material of the sandstone is entirely absent from 

 the shales. On Point Sal it was a change from beds of clay which 

 in all probability were being deposited at no great distance from a 

 shore line. That this change was not to abysmal depths is indi- 

 cated, not only by the conformability with the beds below, but by 

 the abundant organic remains which have given rise to the immense 

 quantities of bituminous matter found more particularly in the cal- 

 careous portions of the strata. 



The less silicious portions of the shales at Point Sal are distinctly 

 seen to be made up of organisms of a calcareous nature. In the 

 thin section no definite forms are visible. Scales and fragments of 

 fish occur everywhere except in the most silicious rocks. The only 



