PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 269 



Aporhyolyte. Diabase.] 



also has a few porphyritic crystals which are fresher than the reddish phenocrysts, 

 are gray and distinctly polysynthetically twinned. 



M/c. None of the gray feldspar phenocrysts are cut by the section. There are, 

 however, several of the smaller red phenocrysts. These are much altered and 

 reddened, and show no twinning striag, but some of them are so much altered that 

 traces of this twinning would probably not remain, even if it was once present. 

 The rest of the section is composed of feldspar, quartz, hornblende, augite, magnetite, 

 hematite and chlorite. 



The feldspar is somewhat cloudy, is in small grains and stout crystals, and does 

 not usually show twinning striae, although simple twins are seen. This feldspar is 

 somewhat altered and reddened, is in such small grains, and does not show cleavage 

 cracks, that no careful determination can be made. It, however, extinguishes approx- 

 imately parallel to the long sides of the crystals, and also parallel to the twinning 

 line, when one is present, and it is thought to be largely orthoclase. 



The quartz is in small grains and is sometimes intergrown with the feldspar to 

 form micropegmatyte, and is also in small poikilitic areas. 



The hornblende is the usual green variety and is quite abundant. It occurs in 

 small plates, pleochroic in shades of green and yellowish, and seems to be all of a 

 secondary nature. In fact, in some places, it can be seen as an alteration from 

 pyroxene. 



The augite is greenish to colorless, and is usually in small grains, many of which 

 are altered to hornblende. There are a few larger crystal of augite, partially idiomor- 

 ph'ic, which are of about the same size as the red sub-porphyritic feldspars of the 

 section. 



The rock is considerably altered, and much of the quartz is probably secondary. 



One section. 



Aye. Cabotian. 



Eennn-lis. This rock and No. 224 are evidently the same, petrographically, and 

 in age, and are thought to be parts of the same mass. Their present condition does 

 not clearly show their original nature, but it seems probable that they were orig- 

 inally of the nature of rhyolytes or trachytes. The small amount of quartz, much of 

 which is secondary, would perhaps cause these rocks to be more accurately termed 

 apotrachytes, but in accordance with the classification of the other rocks of a similar 

 nature and age, they are here referred provisionally to the aporhyolytes. u. s. G. 



No. 227. DIABASE (?) (Po>-/>hijritic.) 



Same locality as No. 226. A narrow dike-like form, not sharply separated from No. 226. " It is narrow 

 and its line of bearing becomes confused, or blends with the rock No. 226, being perhaps a modified form only of 

 No. 226, due to different influence in upheaval, or to unseen contact with accompanying igneous rock." 



Kef. Annual Report, ix, page 57. 



