PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 168 



Porphyryte. Prehnite. Diabase.] 



No. 79. PORPHYRYTE. ( Amygdalmdal. ) 



Eastward from Crystal bay, similar rocks extend for a mile or more. These are brownish or black and 

 basaltiform outwardly, and are doubtless a continuation of No. 78, though exhibiting in the field considerable 

 variation of dip and of structure. They are sometimes compact and sometimes amygdaloidal, and are associated 

 with fragmental rock in a confused manner, the latter being brecciated, and transversely and falsely bedded 

 like some sandrocks. The bluffs are generally low, three to ten feet, with interruptions. 



Ref. Annual Report, ix, page 23. 



No section. 



AIJC. Cabotian. N. H. w. 



No. 80. PEEHNITE. 



From the old copper shaft about a mile up French river. This mineral tills and lines numerous cavities of 

 various ramifying shapes, in the rock No. 81 . Rolled pieces of this mineral are found in the gravel of the beach, 

 and metallic particles of copper are frequently embraced in them.* 



Ref. Annual Report, ix, page 23. 



Mr;/. This prehnite. is associated with another form of gaugue, both carrying 

 metallic copper. This form of the gangue is a siliceous rock of a light, reddish-brown 

 color, apparently fragmental, which, according to the field descriptions, runs in irreg- 

 ular veins and crevices in No. 81. It is hard, like a quartzyte, and dense, but it 

 plainly embraces fine grains of a darker mineral than quartz. The rock has a color- 

 banding which is somewhat flexuous, simulating a fluidal structure, suggesting the 

 silicification and other devitrification of a glassy eruptive. 



M/r. On making a thin section of this peculiar gangue rock, it is seen to be a 

 fragmental rock, consisting very largely of rounded fragments of prehnite, quartz, 

 epidote(?) and of devitrified glass, with an abundant deposition of secondary quartz. 

 Some of the glassy fragments are hardly devitrified, but remain dark constantly 

 between the nicols. They occasionally take the forms of elongated shreds and 

 strings, and show a rhyolitic structure. 



Two sections. 



.I//'-. Cabotian. 



HaiKirk. This rock seems to illustrate, in its origin and manner of occurrence, 

 the clasolyte of Wadsworth. (Report of the State Board [Michigan] of Geological 

 Survey, for the years 1891 and 1892, page 130). N. H. w. 



No. 81. DIABASE (with olivine). 



Banks of French creek, from the lake shore northward for about half a mile or more. 

 Kef. Annual Report, ix, page 23. 



A dark, fine-grained, heavy rock. 



Mir. Ophitic, with aiiy'de and fel<lx[><n-, the <>lirhn' grains being fine and of 

 a reddish brown color. Biotite is also brown and quite abundant. Some of the 

 vesicles are filled with oilritc and some with chlorite of the variety />r/ntinr, judging 

 from the bluish interference color between crossed nicols. 



One section. 



*For an account of this mining enterprise, < Minn<-sni<i llixinrirul < w/.v/i'., us. vol. ii, p. 181, by H. M. RICK. 



