412 THE GEOLOGY OP MINNESOTA. 



[Basalt. Diabase. 



No. 541. BASALT. ( /Arkelyte. ) 



Grand Portage island. A dike ten feet wide cuts the c|iiartzyte and conglomerate, running E. 10 N. by 

 needle, without displacing the bedding. 



Annual Report, x, pages 45, 46. 



Very fine-grained, nearly black. 



The rock is fresh and undecayed. The ijlaxsy n^nlinnu is not abundant, 

 but clear, although it embraces some inni/iirfitc rods and some polarizing crystalliths. 

 The aiu/ite is in fine, isolated, roundish grains that antedate the feldspar for the most 

 part, but which are occasionally broken by the feldspar, showing that these minerals 

 were nearly cotemporaneous. 

 One section. 



. (i/e. Manitou. N. H. w. 



No. 542. DIABASE (with oliviiie). 



Grand Portage island; on the east side of the island this is basaltic, rising about twelve feet. Stratigraph 

 ically it is the highest rock on the island, having a thickness of fourteen feet. 

 Rcf. Annual Report, x, pages 45, 46, 47. 



Mn/. A gray, rather coarse-grained rock. 



Mir. The/efcfepars are ophitically related to the augite. The oil chic is rather 

 fresh. There is a residuum of brownish substance, evidently non-differentiated base. 

 One section. 



Age. Manitou. N. H. w. 



No. 543. BASALT. (Zirkelyte.) 



Grand Portage island. Immediately underlies the last, having a thickness that may be twenty feet. 

 Ref. Annual Report, x, pages 45, 46, 47. 



The rock is amygdaloidal with light blue chalcedony (or with quartzine) 

 and with calcite. 



Mic. The feldspar is partly ophitic in the augite and partly later than the 

 augite. It is also similarly surrounded by the basaltic residuum. Olivine is rather 

 fresh, but not abundant. The basaltic residuum has changed partly to a brown sub- 

 stance in which some small feldspars and occasionally apatite are visible, and partly 

 to minutely microlitic substance, in which the microliths are not spicular but round, 

 and polarize like mica. They are densely packed. This kind of alteration is 

 occasionally mingled with the brownish or greenish product already mentioned, but 

 in several conspicuous areas it surrounds in narrow belts a greenish fibrous substance 

 whose spicules are spherulitically placed, but which does not easily transmit light. 

 The nature of this greenish fibrous substance is not determined, but it may be thalite, 

 as its hardness is about the same as saponite, of which thalite is a form. 



One section. 



Age. Manitou. x. H. \\ . 



