252 THE GEOLOGY OF MINNESOTA. 



[Diabase. 



Cabotian, and that the basal conglomerate of the Potsdam is beneath the Good 

 Harbor sandstone. N - H - w - 



No. 198. DIABASE (ivith olivine). 



From near the mouth of Fall river, on the west side of the river, at the point illustrated in Xorwood s 

 report; apparently continues to Grand Marias; at one place overlies an amygdaloidal red rock. 

 Ref. Annual Report, ix, page 52. 



Meg. This rock has a basaltiform structure and a coarse grain, and brownish- 

 gray color. 



Mir. Section showing a feldspar cut parallel to the brachypinacoid, has extinc- 

 tion at 38, and the positive bisectrix (??) is also nearly perpendicular. This indi- 

 cates bi/toirnite. The field is largely occupied by <>lirhi<' in broken and often small 

 fragmental grains. The augite, which must have been of later generation, is much 

 obscured by dark impurities. Indeed, in some cases, it appears never to have formed 

 as augite. The magmatic remnant, after the magnetite, bytownite and the olivine 

 had crystallized out, appears to have congealed without complete differentiation, yet 

 it was not in the condition of glass at the time of solidification, since these obscure 

 areas exhibit an imperfect double refraction, with recurring extinctions. 



One section. 



Age. A Cabotian flow from the main gabbro mass, analogous to the Beaver 

 Bay diabase, of which it may be the equivalent. N. H. w. 



No. 199. DIABASE (loith olivine). 



From the basaltic columns at Grand Marais (No. 536). 



Ref. American Association for the Advancement of Science, xxx, page 163; Annual Report, ix, page 52; 

 Annual Report, x, page 139. 



Meg. Rather coarse, brown to black, basaltic. 



Mir. The characters of this rock are almost identical with those already assigned 

 to No. 198. It differs from that only in the possession of distinctly pure attgite, which 

 embraces the feldspars and the oli vines, as well as non-differentiated remnants of the 

 magma. The feldspar also appears zoned in one large crystal, in the centre of which 

 is a large accumulation of chloritic impurities. 



Two (thick) sections. 



Age. Cabotian; perhaps the Beaver Bay diabase. N. H. w. 



No. 200. DIABASE (with olivine and copper). 



" Samples of copper-bearing greenstone (gabbro), from N. W. % sec. 24, T. 61-1 W., up Fall river. This 

 heavy-bedded rock has slickensided seams, or thin filling between layers. These seams contain much chloritic 

 mineral (delessite?),some layers of it being one-half inch thick, with stilbite closely mixed with it, and also small 

 quantities of calcite; the copper occurring in the massive, hard greenstone, or doleryte, in the form of thin 

 spangling sheets once or twice the thickness of paper, or even one-quarter inch thick. The sheets sometimes 

 embrace three or four square inches in area. This location was wrought by Johnson and Maguire in the sum- 



