280 THE GEOLOGY OP MINNESOTA. 



[Slate. Gabbro. Conglomerate. 



Age. These beds are Manitou, lying above the conglomerate, which is in outcrop 

 a little further north in the hills west from Grand Portage bay. N. H. w. 



No. 252. SLATE. 



From the Animikie at the village of Grand Portage. The outcrop is near the water along the beach, 

 rising also into short hills a short distance inland. 

 Kef. Annual Report, ix, pages 61, 62. 



Meg. Rather hard and gritty; the slatiness is due to the sedimentation, with 

 occasionally a clay-ball impression. 



Mic. Principally of quartz and feldspar, in angular and sub-angular fragments, 

 with a rare scale of Muscovite. Some of the feldspar is scantily banded with albite 

 striations. 



One section. 



Age. Taconic. N. H. w. 



No. 253. GABBRO. 



From a dike at Grand Portage, near the village, cutting the slates. This dike is thirty-nine feet wide and 

 runs E. 15 S. 



Ref. Annual Report, ix, page 61; Bulletin ii, page 112. 



Meg. Apparently the same kind of rock as No. 248. 



Mic. The composition and grain of this rock are identical with those of No. 248. 

 One section. 



Age. Manitou(?) N. H. w. 



No. 254. CONGLOMERATE. 



Prom the base of Portage Bay island, on the northeastern side. 



Ref. Annual Report, ix, page 61; Annual Report, x, page 34; American Geologist, xiii, page 437, June, 1894. 



Meg. Rather coarse, firm, quartzose conglomerate, but containing fragments 

 of a gray, slaty rock, resembling the slate of No. 252. This lies in large fallen pieces 

 on the shore. These masses are evenly stratified, and show false bedding. The dip 

 in situ is S. 10 E., at an angle of 8 to 10. It shows at least twenty feet, and is cut 

 by a dike nine feet wide. Its color is, in general, gray, but it is spotted sparingly 

 with red pebbles, which can be referred, with the slate fragments, to the hills of the 

 mainland adjacent to this bay, where the red rock of the region appears. It also 

 holds gray quartzyte and flint. 



Mic. This conglomerate, consisting largely of vitreous quartz grains, yet 

 contains some grains of a triclinic feldspar showing the albite twinning, and of 

 microcline with its characteristic quadrillage. The cement is calcite, but this calcite 

 is so abundant that it serves more than as a cement, and occupies independent areas 

 as if it were rapidly accumulating when the conglomerate was being formed. 



In some parts of the thin section examined, indeed in a large part of it, calcite 

 is not so abundant, but in its place, apparently, is an isotropic, greenish, fibrous 



