74 HAROLD W. FAIRBANKS 



tive sheet, the inclosing strata have been much disturbed and 

 show locally a marked variation in strike and dip. 



The remnant of a supposed laccolite roof on Hunters Point 

 I would interpret to be a body of jasper and sandstone or shale 

 inclosed in the serpentine, as a portion of it occupies a sag 

 between higher serpentine ridges, and apparently extends down 

 into a small ravine. 



It is fully as difficult to believe that the Fort Point occurrence 

 is a laccolite. The stratum of shale, sandstone, and occasional 

 bodies of jasper, which appears in the cliffs and along the beach 

 south of the fort, dips into the cliffs at an average angle of 

 not less than 30°, and it seems to me that it can be nothing 

 else than an inclusion between two sheets of serpentine. 



The exposures at the Potrero are good and bear out the 

 opinion which I have already expressed. 



That the two dominant ridges, Montara Mountain and San 

 Bruno Mountains, are fault blocks of such importance, or their 

 diastrophic history so clear as Professor Lawson outlines it, does 

 not seem evident to me. The San Bruno Mountains are said to be 

 the older block, but the southern slope facing the supposed fault 

 is remarkably bold and steep, not showing an advanced stage of 

 degradation, and in addition the northern slope is almost as 

 abrupt. The supposed fault of 7000 feet appears almost incred- 

 ible, and it does not seem at all necessary to postulate it in 

 order to account for the position of the Merced series. That this 

 series should once have existed over the whole of the northern 

 end of the San Francisco peninsula appears very problematical 

 at least. That a series over a mile in thickness, and of so late 

 an age, should have been so completely removed as not to leave 

 a trace north of the supposed fault it is not easy to believe. 



I cannot find any evidence either of the supposed great fault 

 on the southern slope of Montara Mountain. The topography as 

 a whole does not support the idea, and that the ridge is a simple 

 tilted block is not supported by the evidence which Professor 

 Lawson adduces of important faults on the northern slope. Pro- 

 fessor Lawson's suggestion in another place that Montara Moun- 



