BASALTS. 157 



and foriiLs pretty well develujx'd volcanic cones of small size. The scori- 

 aceous basalt does not differ from the ordinary variety under the microscope, 

 except by the pre.'ience of many cavities and a comparatively large amount 

 of ferrujrinous decomposition-products. In this region olivine is very ir- 

 regularly distributed, some croppings showing unusually great quantities 

 of this mineral, while in others it is macroscopically and microscopically 

 absent. This irregularity was noticed even in single patches of the rock, 

 which could not possibly be assigned to different eruptions. Whether 

 olivine is absent or present, the structure of the rock remains the same, the 

 interstices between small, lath-like plagioclases being filled with pyro.xene 

 microlites. 01ivine.is thus a frequent but not an essential constituent of 

 the California basalts, and the microscopic distinction between this rock 

 and andesite consists in the order of genetic succession of the component 

 minerals. The olivine frequently includes quadrilateral picotite crvstals 

 and as a rule shows signs of incipient decomposition. 



The feldspars of the basalts of the ([uicksilver belt are seldom porphy- 

 ritic and they usually assume the form of elongated, more or less microlitic 

 crystals, often with terminal faces. The predominant species is labradorite, 

 but oligoclase may be present among the smaller individuals. The pre- 

 dominant pyroxene is augite, which is sometimes present in large grains, 

 but always as small grains in the groundmass. It presents no peculiarities. 

 Hypersthene is also found in a few sections. It of course extinguishes 

 light when the principal nicol sections are parallel to the main axis, and it 

 shows interference colors which are gray and yellow, differing markedly 

 from those of augite. Like the hypersthene of the Chalk Mountain ande- 

 site, however, its dichroism is not sensible. So far as ascertained, only the 

 larger pyroxene crystals are ever hypersthene, that of the groundmass 

 where determinable being augite. Hypersthene has been supposed by 

 Messrs. Hague and Iddings to replace olivine. To some extent this appears 

 to be the case in the basalts of the quicksilver belt; at least there are a 

 number of thin sections in which hypersthene occurs and which do not 

 contain olivine. Olivine and hypersthene also occur in the same sections, 

 however, and there are many slides in which neither olivine nor hypei'sthene 

 appears. 



