5/8 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



dense in the areas where it is best developed. This forest is composed 

 almost exclusively of Pinus muricata, which is accompanied by a little 

 Pisania densiflora and Quercus agrifolia and by considerable Umbellu- 

 laria californica on very steep slopes. The densest part of the forest is, 

 however, pure Pinus muricata. The shrubs of the Point Reyes Peninsula 

 are northern types, which have here their southernmost or nearly their 

 southernmost representation. These shrubs include Rubus spcctabilis 

 maiziesii, Ledum glandulosum and Rhododendron calif omicum. 



The differences in the floras of these two regions are in 

 part due to different soils produced by the contrasting forma- 

 tions, but since the plants on the triangle are northern types 

 it seems probable that actual geographic separation may like- 

 wise be a potent cause. The triangle may have been an island 

 during late Pleistocene time and this relict flora may be due 

 to the protection secured by this separation from the main- 

 land mass. 



A comparison of the present faunas of these two regions 

 might lead to some interesting results. 



THE RIFT VALLEY OF TOMALES BAY 



As the geologic map shows, Tomales Bay and the valley 

 of Olema Creek form a very distinct zone separating the 

 mainland mass from that of the Point Reyes Triangle. 

 Tomales Bay is essentially a graben between these two blocks. 

 This graben is not a simple one as the Pleistocene deposits 

 in the northeastern side of the bay show. ( See Plate XX ; Plate 

 XXXIV, Figures 1 and 2.) The Millerton formation of 

 lower or middle Pleistocene age as is indicated upon the map 

 of this region has been broken into several different wedge- 

 like blocks. Since this region has been carefully studied in 

 relationship to the San Andreas Fault in the Report of the 

 California Earthquake Commission, 28 the reader is referred to 

 this report for further detail. 



BERKELEY HILLS BLOCK 



The Block described under this heading comprises the 

 northeastern portion of region discussed in this paper. The 

 writer believes that this is a definite orographic block which 

 during the Tertiary has moved as one mass at times, but at 

 other times has been broken into subordinate blocks of notable 

 size. 



28 State Earthquake Commission upon the California Earthquake of April 18, 1906, 

 Carnegie Institution of Washington, Vol. 1, pt. 1, pp. 65-91; pp. 30-35, 1908. 



