364 



University of California Publications. 



[Geology 



COUNTRY IN WHICH THE VEINS ABE FORMED. 



As already stated, the chief rock of the surrounding country 

 is serpentine. This is of a type common in the Coast ranges and 

 in general derived from the alteration of a peridotite. Small 

 areas of a pyroxenic facies occur. Nowhere so far as known do 

 the veins under discussion occur in actual contact with the ser- 

 pentine, although it surrounds the deposit and is frequently not 

 many yards distant from them. 



The rocks immediately associated with the veins are all more 

 or less altered, and this alteration is greatest close up to the zone 

 of veination. In the less altered parts both igneous and sedi- 

 mentary types are recognized. The more common type has in 

 the field the usual appearance of the Franciscan greenstones. 

 Under the microscope it is seen to have originally possessed a 

 diabasic structure. In some specimens the augite is still largely 

 intact. The feldspars however are recrystallized into a fine 

 granular mass. Yet they often show very clearly by the outline 

 of the granular areas the lath-shaped forms of the original feld- 

 spars and the relationship to the augites that characterize the 

 diabase structure. Some titanite is present. In a somewhat 

 altered specimen the augite is more or less altered into chlorite, 

 while in the feldspathic layers small greenish or bluish needles are 

 commencing to form in some cases actinolite, occasionally 

 glaucophane, or some other geologically related amphibole. The 

 new feldspar is at least in large part albite. 



On the south hillslope below the east end of the deposit is a 

 spheroidal gabbro. The grains and prisms of monoclinic pyro- 

 xene are in part altered to chlorite. The labradorite is more or 

 less decomposed and otherwise altered and the rock is impreg- 

 nated with calcite. It does not come in contact with the veins at 

 any point. 



Other rocks are found having the characteristics commonly 

 displayed by the more altered Franciscan sandstones or grey- 

 wackes. Under the microscope the light colored constituents 

 which make up the bulk of the rock are seen to be entirely re- 

 crystallized into very fine granular aggregates. The original 

 structure is preserved by the dark films of ferruginous or car- 



