-910.] COAST RANGES OF CALIFORNIA. 319 



The second belt has an east-and-west trend, and follows for 

 about three miles the ridge that forms the Colusa and Lake County 

 dividing line, then extends one mile to the west. 



Of the two smaller areas one is found lOO yards northwest of 

 the Wilbur Spring Hotel, the other is best located by the Wideawake 

 quicksilver mine. Both aggregate only a small fraction of a square 

 mile, but the former has attained a certain local fame, due to the 

 abruptness with which it terminates, exposing an almost perpen- 

 dicular height of about lOO feet, which is known as the lovers' leap. 



With the exception of the last the serpentine outcrops are very 

 low. They seldom protrude more than a few feet above the sur- 

 rounding soil, the removal of which keeps pace with the erosion of 

 the rock. All stages of decomposition can be observed. Very 

 fresh rock is found where Sulphur Creek cuts the first named 

 belt of serpentine. It is exceedingly brittle, of a dark green color, 

 massive, and shiny individuals of pyroxene give it a mottled ap- 

 pearance. As it weathers it assumes a brownish color, and finally 

 gives rise to a brown soil. 



Thin sections from fresh serpentine show a colorless mass with 

 a slight green tinge with now and then patches of a more intense 

 green, the whole somewhat pleochroic. Crossed nicols reveal ser- 

 pentine to be the predominating mineral, with enstatite, diallage and 

 olivine less abundant. ]\Iinor quantities of picotite, chromite and 

 magnetite are present. 



The microscopical investigation was supplemented by a chemical 

 analysis made by the writer in the mineralogical laboratory of Stan- 

 ford University, of fresh serpentine from the bed of Sulphur Creek, 

 with the following results : 



The analysis shows a very basic rock, and points towards a peri- 

 dotite. It corroborates the microscopical investigation. Using the 

 theoretical formula for serpentine the approximate ratio of serpen- 

 tine to other minerals present can be calculated by assuming that all 

 the water goes with the serpentine. The ratio thus obtained is four 

 to one. 



The above presented evidence shows that the parent rock was an 

 eruptive, very basic, and contained the minerals which define a Iher- 

 zolite. The serpentine is therefore a Iherzolite serpentine. 



