276 IRVING. 



3. Foley Peak Type. 



4. Nevada Gulch Type. 



5. Whitetail Gulch Type. 



Portland Type. 



At the head of Squaw Creek, near the old Portland mill, 

 occurs a very thick sheet of quartz-porphyry. 



Megascopic Cliaracter. — It is a reddish brown rock, much de- 

 composed, and only in rare instances shows macroscopic crys- 

 tals of quartz. The groundmass is brown in color and exceed- 

 ingly dense, containing occasional flakes of biotite. The 

 feldspars are of uniform size, averaging about a fourth inch in 

 diameter, and are so much resorbed that they often show a 

 somewhat lenticular aspect, which has given rise to the local 

 name of " Bird's Eye" porphyry. 



Microscopic CJiM^acters. — Under the microscope the rock is 

 seen to consist of phenocrysts of sanidine and plagioclase em- 

 bedded in a groundmass of very fine xenomorphic grains of 

 quartz and orthoclase. 



The sanidines are somewhat resorbed and are prevailingly 

 twinned after the Carlsbad Law. They far exceed the plagio- 

 clase crystals, both in size and number, but the latter attain a 

 somewhat unusual development for a typical rhyolite. Pheno- 

 crysts of microcline are also of frequent occurrence. Biotite 

 is scattered through the groundmass in quite conspicuous 

 masses. The rock is one of the typical quartz porphyries, but 

 is usually taken for trachyte, as the quartz is not apparent to 

 the eye. It differs from the other quartz porphyries in the ab- 

 sence of microscopic quartz, and the rounded character of the 

 phenocrysts. 



