,907.] CRANDALL— SAN FRANCISCO PENINSULA. 17 



amount of movement that has taken place, due to the increase in 

 volume in the formation of serpentine.^ 



The peridotite is the fresh facies of the serpentine, and has 

 been shown petrographically to be the rock from which the serpen- 

 tine proper is derived. The other or slickensided facies of ser- 

 pentine is the one with which nearly everyone is familiar. It is 

 blue to green in the hand specimen, soft and smooth to the touch. 

 It flakes roughly, giving always new slickensided faces showing 

 that there have been movements in every little particle of the rock. 

 There are occasionally varying streaks of a hard brown rock that 

 seems different from the main mass but which petrographically is 

 just the same. 



Associated with these serpentines are small veins of magnesite, 

 some of it of the fibrous variety, and some hard and compact. 

 Another rock that is found with the serpentines has been termed 

 " silicious carbonate sinter " by Professor Lawson. 



This material is a weathered product of the serpentines. It is 

 a hard, light-colored rock showing lines of silicification throughout. 

 It is believed to have been produced by a leaching of the mag-- 

 nesium, in the form of a carbonate, from the serpentine, with a 

 simultaneous redeposition of the silica present as amorphous silica, 

 and more or less stained with the iron from the original rock.- The 

 magnesium leached out probably goes toward the formation of the 

 magnesite veins common in the serpentine. Rock sections show 

 but little except the lines of silicification, as the amorphous silica 

 does not react in the ordinary or polarized light. 



Petrography of the Serpentines. 



The petrography and a few chemical analyses of the serpentines 

 that are available are of much interest.^ The description of two 

 facies of the serpentine will be given, the massive facies and the 

 slickensided facies. 



^ G. P. ]^Ierrill gives 22 per cent, as the increase of volume due to forma- 

 tion of serpentine by the hydration of olivine. 



" Knopf, A., An Alteration of Coast Range Serpentine, Bulletin Depart- 

 ment Geology University California, IV, No. 18, p. 425. 



^ Dr. Palache has described the serpentines in his paper on the Ihezolite 

 serpentines of the Potrero. Bulletin Department of Geology University Cali- 

 fornia, I, No. 5, p. 235. 



