Ransome. ] 



Eruptive Rocks of Point Bonita. 



73 



GENERAL ACCOUNT OF THE GEOLOGY. 



Reference to the accompanying map will show the general dis- 

 tribution of the several rocks. It will be seen that a considerable 

 portion of the area is covered by comparatively recent deposits, 

 represented by dotting, and the mapping of the underlying forma- 

 tions must be taken as more or less conjectural when so concealed. 

 The eruptives, with the two small patches of pyroclastic rock, form 

 the mass of Point Bonita proper, and extend in an almost unbroken 

 belt of varying width up to the sand-bar at the lagoon. Imme- 

 diately to the east occurs sandstone, which is identical with that so 

 extensively developed on the San Francisco Peninsula and about 

 the shores of San Francisco Bay. Although generally regarded 

 as of Cretaceous age, its true position cannot be considered as finally 

 settled, and it will therefore be spoken of as the San Francisco sand- 

 stone, in accordance with the usage of Blake.* At the point marked 

 G, the sandstone is distinctly bedded, and dips north 25 ° east, at 

 an angle of 55 . The contact is defined by a little ravine, but the 

 actual junction of the two rocks is concealed. Farther north, near 

 C and D, two small patches of the sandstone are seen resting upon 

 the eruptive rock, the latter being intrusive. 



Succeeding the sandstone on the north and northeast occurs a 

 band of the very characteristic dark-red bedded jaspers described 

 by Blake.f Newberry, J Whitney ,§ and Becker,|| and named "phtha- 

 nites" by the latter. The eruptive rock has been intruded into 

 these jaspers, and they are disposed in great confusion along the 

 upper portions of the cliffs, with a general tilting to the east. 

 Where less disturbed their dip is such as would indicate that they 

 pass under the sandstone to the southwest. Sandstone again occurs 

 in the northeast corner of the map, and, although not sufficiently 



* Reports of Explorations and Surveys for a Railroad from the Mississippi 

 River to the Pacific Ocean, Vol. V, House of Rep., 33d Congress, 2d session, 

 Ex. Doc. No. 91, Geol. Report by W. P. Blake, pp. 145-154, Washington, 1856. 



■fLoc. cit., pp. 155, 156. 



J Report of Explorations, etc., Vol. VI, Senate, 33d Congress, 2d session, 

 Ex. Doc. No. 78, Geol. Report by J. S. Newberry, p. 12, Wash., 1857. 



I Geological Survey of California, Geology, Vol. I, p. 82, 1865. 



|| Quicksilver Deposits of the Pacific Slope, Monog. XIII, U. S. Geol. Surv., 

 pp. 105-108. 



