Vol. XI] D1CKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 537 



follows: "In the field the granites of Point Reyes appear as 

 moderately coarse-grained, light gray rocks showing rough 

 and rounded surfaces where they are firm, though usually 

 they are much decomposed. Where erosion is not rapid the 

 rocks are decayed, often to a depth of a dozen feet or more, 

 but on the summits where harder phases protrude, and 

 in the deep ravines where the erosion is greatest, and 

 along the shore the rocks are firmer and often more angular. 

 All of them, where favorably exposed, are seen to be greatly 

 shattered and broken, and testify to the large amount of 

 disturbances they have undergone. The rock is mostly unfit 

 for quarrying purposes on this account, since it is not easy 

 to find many blocks of any considerable size. These granites 

 are to be classified as normal biotite granites. Quartz is in 

 only moderate proportions, and both orthoclase and plagio- 

 clase feldspars are present. As to the quantity of biotite 

 present, there is considerable variation. Basic segregations 

 are common, in which there is no quartz and little feldspar, 

 while on the other hand there are phases containing but little 

 biotite. Hornblende is not abundant." 



Franciscan Group, Jurassic (?) 

 Rocks of this group are restricted to the mainland in con- 

 trast with the granitic rocks of the Triangle. The San An- 

 dreas Rift Zone sharply separates these terrains, but unfor- 

 tunately at no place are they in close contact. There can be 

 no doubt that they were separated by ancient movements 

 along the San Andreas Rift. This ancient fault line seems 

 to lie on the western side of Tomales Bay, as the Franciscan 

 occurs beneath the Pleistocene deposits on Hog Island and 

 Tom's Point, both of which are located about two-thirds of 

 the way across Tomales Bay from its eastern shore. The 

 eastern face of Inverness Ridge must of necessity be inter- 

 preted as a much eroded fault scarp. The Franciscan group 

 occupies the greatest area of any in this field and it under- 

 lies most of the region now covered by the Merced formation. 

 Island-like masses such as Meachims Hill, appear through the 

 veneer of Pliocene sediments in many places. In other words, 

 the peneplain upon which the Merced was deposited was cut 

 in Franciscan rocks, and many monadnocks were left stand- 

 ing above the general surface of this notable feature. In and 



