PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



421 



Diabase. Conglomerate.] 



Mic. This rock has the same characters as No. 558, of which it is probably a 

 portion. This, however, contains, in the hand specimen, no amygdaloidal structure, 

 but it is permeated with a serpentinous, greenish mineral whose hardness is about 

 2 or 3. This is yellowish-green in small masses, but is dark when thick. It appears 

 to be thalite, having its vermicular structure and positive elongation. One section. 



Age. Manitou. N. H. w. 



No. 560. DIABASE (with olivine}. 



Forms a bed under the cupriferous conglomerate No. 557; known as the "greenstone range," where it rises 

 to the surface further north. 



Kef. Annual Report, x, page 51. 



.If///. Gray, medium-grained, specked with small nests of calcite, hence on the 

 weathered surfaces somewhat porous, made up of a plagioclase and a rusty brown 

 mineral, the product of alteration of either olivine or augite. Other ingredients are 

 indistinct. 



Mic. The rock is ophitic, but the augite is nearly lost by alteration, its form 

 only remaining as a rusty, indefinite substance, embracing the feldspars. The olivine 

 (which was in part later than the feldspars) is altered to the brown substance men- 

 tioned, which is only brown by reason of accumulated oxide of 

 iron, being transparent in the centre of the grain, and even 

 almost isotropic. These parts are as shown by the accompany- 

 ing sketch, in which a represents the usual and familiar isotropic 

 product of olivine alteration when not rendered opaque by 

 iron. Extinction in the grain, especially visible in the part c, 

 and faintly in d, is parallel with the longitudinal cleavages. 

 The transverse cleavage in the section sketched is not precisely 

 perpendicular to the longitudinal. In another grain, however, 

 these cleavages are perpendicular to each other. This is the 

 substance which has been referred to as bowlingite in several . 

 rocks. It seems to be monoclinic. 



The fclhpar is much altered and multitudes of highly 

 refractive grains are crowded in it, some of which are zoisite, 

 having a longitudinal extinction. 



The tntgite is plainly preserved, but much destroyed. 



Age. This is apparently from a Cabotian intrusive series rising in the form of 

 grand dikes forming the most of the northern precipitous shore of Isle Royale. It 

 is comparable to the dikes in the region of Grand Portage. N. H. w. 



No. 561. CONGLOMERATE. ( Finer debris. ) 



FIG. 25. CLEAVAGE OF 

 BOWLINGITE IN ROCK 



NO. 560. 



From the cupriferous part of the conglomerate at the Island mine, Isle Royale. 

 Ref. Annual Report, x, page 51. 



