Vol. 4 i Osmdnt. — Geological Section of Coast Ranges. 



63 



contact. It is dark greenish black, heavy rock, rather too coarsely 

 crystalline for a basalt, with scattering phenocrysts of feldspar. 



Microscopically it is also similar. It is somewhat fresher, 

 and contains much less hematite. By Michel Levy's statistical 

 method the feldspars gave a maximum angle of 43.5° This indi- 

 cates a basic labradorite of a composition somewhat more basic 

 than Ah, An 4 or nearly Ab, An 2 . Augite ocenrs sparingly as 

 phenocrysts up to .38 mm. in length. These crystals are rounded 

 and corroded as though acted upon by the magma prior to con- 

 solidation. The abundant angite in the ground mass occurs in 

 rounded grains lying between the laths of feldspar in the "Inter- 

 sertal" structure of Rosenbusch. The feldspar lathes are short 

 and stout, and invariably twinned on the albite law. The augite 

 is of the usual lavendar gray color. It appears to be altering to 

 chlorite of a dark green shade, which stains the rock freely. No 

 olivine was observed in this slide. A slight flow structure was 

 observed. No glass was recognized. A careful determination of 

 the silica contents of this rock gave 65.13%. 



A specimen from below the Sonoma Tuff on the southwest 

 slope of Mt. St. Helena may be described as follows : 



Macroscopically it is a dark grayish black rock, containing 

 phenocrysts of lath-shaped feldspars and irregular greenish yel- 

 low decomposed augite crystals in a fine grained ground mass, 

 which does not appear crystalline to the naked eye. 



Microscopically this rock is made up of numerous large and 

 small phenocrysts of feldspar and of augite embedded in a semi- 

 crystalline ground mass composed of numerous feldspar micro- 

 lites embedded in glass. The feldspar phenocrysts are frequently 

 well terminated, but the larger ones are usually broken and show 

 wavy extinction. The maximum angle observed in sections show- 

 ing symmetrical extinctions on (010) was 38°. This corresponds 

 to a labradorite of about the composition Ab 3 An.. The augite 

 phenocrysts are worn, broken and corroded, and usually altered 

 to chlorite and hematite. They were preceded by the magnetite 

 which is found included in them. The feldspar is moulded on 

 the augite. There is much magnetite dust in the glass of the 

 ground mass, and also much chlorite, which stains the rock green 

 in thin sections. This may have come from original augite grains 



