BASALTIC AXDESITE AiS T D BASALT. 303 



in those sections exhibiting little polysynthetic twinning. In a few instances 

 there is a central core of feldspar, first recognized between crossed nicols, 

 and this is invariably more calcic than the marginal feldspar. Grlass inclu- 

 sions are the same as in the feldspars of the pyroxene-andesites. Con- 

 temporaneous crystallization with pyroxene is occasionally observed. 



The pyroxene phenocrysts are augite in nearly all the rocks in which 

 olivine is abundant. In a few cases hypersthene is also present, but in smaller 

 amount than augite. Transitional varieties with hypersthene and olivine in 

 reciprocally varying proportions have been mentioned in connection with 

 the rocks in other parts of this region. The augites have the same appear- 

 ance and characters as in the andesites. The same is true of the hyper- 

 sthene when present. Olivine occurs in very small idiomorphic crystals, 

 yielding rhombic and characteristic six-sided sections. It is in most cases 

 wholly altered either to green serpentine or to the red and orange pseudo- 

 morphs which often result from weathering. In the few cases where olivine 

 is still preserved it is colorless in thin section. The serpentinization is quite 

 normal. Both modes of alteration are frequent, but the green serpentiniza- 

 tion is probably more common. They do not occur by the side of one' 

 another in one rock section. In only one instance was anything like a 

 combination of the two observed. The processes appear to be due to local 

 causes, for it is found that all the varieties, with olivine, that were collected 

 at any one locality bear the same kind of pseudomorphs. 



Occasionally colorless olivine is coated with opaque iron oxide, which is 

 red by incident light. It occurs in minute grains penetrating the crystal in 

 rows normal to the side planes. The shape of the grain suggests that the 

 original oxide was magnetite. In cross sections the coating appears as an 

 opaque border of variable Avidth. In one rock olivine is replaced by calcite. 



The irregular and amygdaloidal cavities in these rocks are sometimes 

 filled with secondary minerals, of which chalcedony and hyalite or opal 

 and several zeolites are the commonest kinds. The first two are often 

 associated together, chalcedony coating the walls of the cavity in a spheru- 

 litic layer, opal filling the central portion. In one of the pyroxene- 

 andesites the chalcedony of the margin passes into fibrous or platy quartz, 

 which fills the center of the amygdule, the two being continuous optically 

 as well as in substance. In some sections through such amygdules the 

 central quartz exhibits undulatory extinction, not very unlike that produced 



