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University of California. 



[Vol. 2. 



itous ocean cliff for its whole length. The Point Reyes Light 

 stands at its western terminus. At the beach, below the govern- 

 ment dwellings, is the most western exposure of the granite. It is 

 here overlain by massive beds of conglomerate, but it rises from 

 beneath these to the summit a little farther to the east, and then 

 forms the remaining three-fourths of the sea-cliff extending east- 

 ward. This narrow strip of granite does not exceed a width of a 

 few hundred feet, except at its eastern end, where it juts into 

 Drake's Bay. This projecting headland is of granite, and from this 

 a narrow strip of granite follows the shore of Drake's Bay north- 

 ward for half a mile. It is then replaced by beds of very coarse 

 conglomerate and sandstone. 



Petrograpliic Character. — 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. 



Veins of aplite occur, and are not uncommon, along the eastern 

 side of the peninsula and at its northern extremity. Pegmatitic 

 rock is sometimes found which, occasionally, is very coarse-grained. 

 These granites are to be classified as normal biotite granites. 

 Quartz is in only moderate proportions, and both orthoclase and 

 plagioclase 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. Horn- 

 blende is not abundant, or, rather, it is almost entirely absent. 



There are three prevailing types of granite, one of which is 

 confined to the southwestern area, and one almost wholly to the 

 northern. 



