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University of California. 



[Vol. 2. 



minerals, thus offering a very interesting field for mineralogical 

 study. The main mass of serpentine is dark green, with often a 

 mottled appearance, due to numerous phenocrysts of original ensta- 

 tite, which show shining bronze-colored cleavage faces. Narrow 

 veins of very compact light green serpentine traverse the dark 

 green boulders, and these are generally accompanied by thread-like 

 veins of finely-fibrous chrysotile, which form a border on each side 

 of the light green. 



Among the secondary minerals, forming the white veins, which 

 so abundantly seam the serpentine, magnesite appears the most 

 common. It is snow white and very compact and hard usually, but 

 where the mass of serpentine is much decomposed, the mineral 

 occurs in small botryoidal forms, and is softer, owing, probably, to 

 an alteration in part to hydromagnesite. Calcite is also abundant, 

 mostly in cleavage masses, sometimes in small scalenohedra. 

 Long and slender colorless prisms of aragonite, white tabular 

 crystals of barite and small gypsum needles are also occasionally 

 found in the fissures. Grayish-brown opal occurs in small rounded 

 inclusions in the serpentine on the east side of the fort. 



A few hundred yards west of the fort a large mass of rock 

 outcrops on the shore of a different nature from the serpentine, and 

 is probably a remnant of an older basic rock included in the serpen- 

 tine. The mass has a foliated or somewhat schistose structure due 

 to shearing, and is highly altered to chlorite, and contains the veins 

 of pectolite and datolite. 



Pectolite. — The numerous pectolite veins are generally very nar- 

 row, sometimes mere white threads, but occasionally they widen out 

 to several inches, the one from which most of the specimens were 

 collected being about four inches in width. The mineral is fibrous 

 and generally very compact and tough, with a more or less radiated 

 structure. In some parts of the vein the fibers are coarser and not 

 so densely compacted. On one specimen short and rather broad 

 prismatic crystals, considerably rounded by wave action, occur. 

 They apparently consist of a broad base <r(ooi), a broad orthopina- 

 coid <?(ioo), and two prisms, perhaps (540) and (140), as indicated 

 by measurements with the contact goniometer. The color of the 

 mineral is snow white, but much of it is stained reddish by varying 

 amounts of ferric oxide. 



