Ransoms 



Eruptive Rocks of Point Bonita. 



85 



Figure 6. — Radial grouping of plagioclases in spheroidal basalt at B. x 35. 



Secondary alteration is here also well advanced, and the plag- 

 ioclases do not lend themselves to accurate measurements of 



The few readings obtainable gave extinction 

 angles of about 17 , indicating labradorite. 



THE DIABASE. 



Occurrence. — As may be seen by turning to the map, this rock 

 occurs in the southern portion of the area, as a narrow, dyke-like 

 band, traversing the spheroidal basalt, the relative positions of the 

 two near the lighthouse being shown in the accompanying section, 

 B' B". As the cliff is here one hundred feet high, and practically 

 vertical, it was not possible to make a close inspection of the east- 

 ern contact, but, looking down from above, it appears to be very 

 irregular, and the pyroclastic rock seems to have been ruptured and 

 displaced. There can be little room for error in regarding it as a 

 true igneous contact, for faulting could hardly have taken place along 

 so irregular a surface without the production of considerable friction- 

 breccia, of which there is here no trace, On the north side of the 

 toe-like projection, upon which the lighthouse stands, the pyroclas- 

 tic rock does not appear, and the diabase can be traced by the eye, 

 as a rather indistinct band alone the face of the cliff. 



