280 IRVING. 



Megascopic Appearance. — It is of a dark bluish gray color^ 

 grading to whitish gray in the more coarsely grained portions. 

 It occurs also in other localities in considerable abundance. 



Microscopic Characters. — The microscope shows the following 

 minerals : orthoclase, plagioclase, quartz, biotite, chlorite and 

 magnetite. 



The groundmass is extremely fine, as in the last rock de- 

 scribed, but is markedly granular in texture. In it are em- 

 bedded in about equal numbers, phenocrysts of sanidine and 

 plagioclase both of which are in an advanced stage of decay. 

 The biotite is in quite noticeable and thickly disseminated flakes, 

 and generally shows a twisted appearance as if disturbed by the 

 flow of the rock. Chlorite and magnetite are also present, the 

 former as a decomposition product of biotite, and the latter in 

 irregular grains. 



The quartz contains inclusions of zircon, and is in the usual 

 resorbed crystals. It also shows an extraordinary amount of 

 fracture, which has evidently been caused by the flow of magma. 

 The fractured portions of the phenocrysts are scattered all 

 through the groundmass, and are of all sizes, giving the rock at 

 first sight the appearance of a breccia. 



The broken edges are generally extremely sharp and angu- 

 lar and only occasionally show a contour rounded by corrosion. 



From the condition of the quartz it is to be inferred : 



1. That the crystallization of the quartz phenocrysts took 

 place before the forces which produced the upward flow were 

 operative. 



2. That the quartz crystals were broken by the flow of the 

 magma during its intrusion. 



Other than the broken quartz crystals, this rock shows marked 

 evidences of flow. They can best be observed in the field. 

 The rock consists of two portions, a granular, white and 

 rather coarse-grained rock of porphyritic texture, and a much 

 darker, at times completely aphantic, material arranged in flow 

 lines or scldicren. The two are mingled together in just 

 such eddies and swirls as are seen in a pot of paint where 

 two colors are mixed together. At one point a large mass of 



