GEOLOGY OF SAN FRANCISCO PENINSULA 69 



thev would occur at all geological horizons. Consequently the 

 correlation of the KnoxviUe with the Franciscan series (Golden 

 Gate series) by this means is out of the question. The sinter is 

 very similar all through the Coast Ranges, and any deposit is a 

 possible source of quicksilver. 



Professor Lawson's discussion of the structure of the older 

 uncrystalline rocks shows, although he minimizes the importance 

 of the disturbances which they have undergone, that it is often 

 difficult of elucidation. The earlier observers have all remarked 

 upon the difficulties connected with a study of the so-called 

 metamorphic rocks, and we cannot admit that all of their work 

 is "superficial." While the author is probably right in asserting 

 that the larger structural features are comparatively simple in 

 the area under discussion, yet it seems to me that there is good 

 reason for believing that the structure is very complex in detail. 

 This is shown by the fracturing and folding of the jasper bands ; 

 the frequent occurrence of crushed shale, and thin-bedded sand- 

 stone in a ruptured condition ; and the presence of innumerable 

 cracks and shear planes in the more massively bedded sand- 

 stones. Local areas occur, it is true, totally free from the effects 

 of strain, but they do not dominate. A comparison of the sand- 

 stone at Point San Pedro with that north of the valley of the 

 same name affords a good illustration. Both in the cliffs and 

 on top of the point the sandstone weathers out in large blocks 

 in a manner closely simulating the sandstone in the Cretaceous 

 and older Tertiary in the Coast Ranges. On the contrary the 

 similar sandstone in the hills north of San Pedro Valley breaks 

 up in angular fragments ; being permeated in many places with 

 veins of quartz and calcite, or linear seamlike cavities. This 

 fracturing and veining is also almost everywhere to be noted in 

 the jaspers and limestone. Professor Lawson considers that 

 because the number of parting planes decreases downward 

 from the surface they are due in great part to atmospheric 

 agencies alone, although he recognizes some shear planes. I 

 believe, however, that we have good reason for holding that a 

 large part of these planes which separate the sandstone into 



