794 THE GEOLOGY OF MINNESOTA. 



[Conglomerate. Tuff. Zirkelyte. Basalt. 



is a fine grained, dark, greenish-black rock looking like a diabase. From the hand 

 specimen it would seem as if this dark rock was included as fragments in the 

 quartz-porphyry, but it may possibly cut the porphyry as an irregular dike. No 

 section. 



Age. Cabotian. u. s. G. 



No. 1831. CONGLOMERATE. 



Northeast side of Grand Portage island, north shore of lake Superior ; S. E. }^ se c- 10, T. 63-6 E. 

 Ref. Annual Report, xxii, page 8. See, also, American Geologist, vol. xiii, pages 437-439; vol. xvi, pages 

 150-162. 



Meg. A coarse sandstone, containing many pebbles, the most important of 

 which are quartz, red granitic rock, quartzyte and gray and greenish slaty rock. No 

 section. 



Age. Puckwunge or Potsdam (bottom conglomerate). u. s. o. 



No. 1832. TUFF. 



East side of Grand Portage island ; the upper portion of the same sandstone which is intercalated between 

 trap beds. 



Ref. Annual Report, xxii, page 8. 



Meg. Darker and finer-grained portion of the sandstone. 



M ic. The quartz grains are entirely angular; and are mingled with a few grains 

 of feldspar and of devitrified glass. These all lie in a brown, rusty background which 

 itself is apparently a devitrified glass of varying grain. 



Age. Manitou. N. H. w. 



No. 1833. ZIRKELYTE. 



Prom the very top of No. 1832, where it is in contact with the overlying trap sheet. 

 Ref. Annual Report, xxii, page 8. 



Meg. Nearly black, fine grained. 



Mic. The specimen seems to have been taken actually from the bottom part of 

 the trap sheet, as it contains no fragmental stuff. It is wholly of the igneous material. 

 It is either glassy and very dark with magnetite, or finely sub-crystalline with the 

 elements of a diabase, amongst which feldspars are distinct, but the augite and 

 olivine have been almost entirely replaced by uralite, calcite and limonite. One 

 section. 



Age. Manitou. N. H. w. 



No. 1834. BASALT. (Globular.) 



Prom the top of Grand Portage island, north shore of lake Superior ; S. E. ^ sec. 10, T. 63-6 E. 

 Ref. Annual Report, xxii, page 8. 



Meg. Same as No. 544. No section. 



Age. Manitou. u. s. o. 



