1907] CRANDALL— SAN FRANCISCO PENINSULA. 47 



deep sea deposits, altered since the time of their deposition. It 

 is reasonable to assume that the older jasper beds may have been 

 formed similarly to those of the ^Miocene age whose origin is known. 



The fact that remains of diatoms do not show in slides of the 

 jaspers does not prove that they were not there originally. These 

 jaspers are mostly amorphous silica and must have passed through 

 a stage of solution and re-deposition from their original form so 

 that all traces of diatoms might easily be lost. 



This hypothesis is worthy of consideration because of the large 

 deposits of siliceous shale that we have in California; and it is 

 more reasonable to consider the older jaspers formed after the same 

 method of later shales, than to assume a different set of condi- 

 tions and different organisms to have existed without any definite 

 proof. The presence of radiolaria does not affect this either way 

 because small amounts of any organism might be present with the 

 diatoms and have their skeletons preserved. 



The fact that in San Francisco we have a lignite and tuffs inter- 

 bedded with the jaspers is significant. This lignite and tuff show 

 conditions different from those under which the jaspers were de- 

 posited. The jaspers represent deep sea or at least quiet water 

 conditions. The presence of lignite shows land conditions, while 

 the jasper pebbles in the sandstone show that the Franciscan area 

 was above the sea and that erosion had set in. 



The tuft's present may mean that a period of volcanic activity 

 was the cause of the change in the conditions under which the jaspers 

 were being deposited and a return to land conditions as shown by the 

 lignites. In this same sandstone are large and small pieces of 

 shale, that resemble the shale of the series of rocks underlying the 

 jaspers. This suggests that erosion went through the entire jasper 

 bed and well down into the San Bruno series. There must be some- 

 where then, an unconformity in the series itself. The extent of this 

 erosion interval is not known nor its place except that one area out 

 of water must have been near the present city, as the pebbles are 

 large and subangular. The second large bed of jasper shows the re- 

 turn of conditions favorable for the deposition of jasper. 



Work through the area south of San Andreas lake and over the 

 Santa Cruz quadrangle shows that the jaspers have been removed 



