Ransome ] 



Eruptive Rocks of Point Bonita. 



107 



is divided into two portions by a formation of bedded jaspers. The 

 southern and larger portion is well exposed along the shore, and 

 afforded good observations for dip and strike. This rock is 

 described at some length by Blake,* Newberry ,f and Whitney. \ 



It is here a fine-grained sandstone of light gray color, in well- 

 defined beds, varying from a few inches to several feet in thickness. 

 The thinner beds are crowded with dark, carbonaceous spots, which 

 are evidently remnants of vegetable matter, lying with their flat sides 

 parallel with the planes of lamination. Thin lenticular sheets of 

 undoubted coal, up to an inch in thickness, were also observed. 

 Beyond these fragments, however, the rock seems barren of organic 

 remains. 



The strike at H is well defined by the series of parallel points, 

 the dip being westerly. Towards K the angle of dip increases, and 

 the beds become thinner and more carbonaceous. At G the sand- 

 stone is somewhat disturbed, but shows a well-defined dip to the 

 northeast, which it appears to maintain without much change until 

 K is reached. At this point all bedding is lost, and the rock shows 

 indubitable signs of crushing and disturbance, indicating a rending 

 or faulting of the strata rather than a simple fold. It is to be noted 

 that of the considerable thickness of strata exposed east of K, but a 

 very small portion again appears between K and G (see Section 

 A' A"). Two or three isolated outcrops of the sandstone occur in 

 the dotted area of the map, and it is seen in conjunction with the 

 ruptive rock near C and D, but is elsewhere concealed by the Ple- 

 istocene deposits. 



The jaspers possess here the very characteristic features which 

 constantly accompany these remarkable rocks about the shores of 

 San Francisco Bay, and in other portions of the Coast Ranges. 

 Those of this particular locality have been described or referred to 

 by earlier observers, as has been already noted. § Dr. Becker has 

 brought forward a theory of metamorphism || to account for their 



*Loc. cit., pp. 145-154. 

 ^Loc. cit., p. 12. 

 %Loc. cit., pp. 76-78. 

 'iAtitr, p. 73. 

 \\Loc. cit 



