298 THE GEOLOGY OF MINNESOTA. 



[Porphyryte. Gabbro. Quartz keratophyre. 



No. 283. PORPHYRYTE. (Diabase.} 



"Prom the larger little island at the west end of High (or Lucille) island." N. W. J4 sec. 8, T. 63-7 E. 

 Ref. Annual Report, ix, page 67. 



Meg, A fine-grained, dark-gray, diabasic rock, with porphyritic plagioclases, 

 some of which are over half an inch across. Some of these phenocrysts show, 

 especially at their edges, a pinkish color; otherwise they are glassy and gray in color. 



Mic. The porphyritic plagioclnses are imbedded in a groundmass which is 

 composed of plagioclase laths in an abundant background of secondary minerals. 

 These secondary minerals are almost entirely chlorite, hornblende and magnetite. It 

 is probable that most of this secondary material, especially the hornblende, originated 

 from augite, although no augite is now present. It is also possible that part, at least, 

 of the areas filled with alteration products, were originally glassy. There are a few 

 green areas which suggest olivines, especially as some of these areas interfere with 

 the outlines of the plagioclase phenocrysts. 



One section. 



Age. Cabotian(?) u. s. G. 



No. 284. GABBRO. (Porphyritic.) 



"From the main dike (?) of High (or Lucille) island, on the south side. The dike itself is horizontally 

 basaltic toward the west end of the island; and a part of the hight of the island is caused by a heavy overflow, 

 but perhaps not from this dike. This dike "hades" to the south, and is a coarse porphyritic greenstone. The 

 samples are from that part that is dike-like." 



Ref. Annual Report, ix, page 67; Bulletin ii, page 118. 



Meg. A fine-grained, gray rock, consisting of small gray feldspars in a darker 

 background. It contains porphyritic crystals of a gray to glassy plagioclase. 



Mic. The section shows but one phenocryst; this is considerably clouded. The 

 groundmass is much altered, even the feldspars being much altered. In structure 

 this groundmass is granitic and is composed essentially of plagioclase and hornblende 

 with some quartz, thus justifying the name quartz dioryte applied to this rock by Dr. 

 Wadsworth (Bulletin ii, page 118). It seems quite evident, however, that the rock 

 was originally a gabbro and is here so-called, the hornblende and quartz being re- 

 garded as secondary. Some iron ore occurs in the section and this in places is seen 

 adjoining a gray opaque substance evidently derived from it; the iron ore is thus 

 ilmenite or titaniferous magnetite, rather than magnetite. One section. 



Age. Cabotian(?) u. s. o. 



No. 285. QUARTZ KERATOPHYRE. 



From the first island northwest of Magnet (or Belle Rose) island. The south side of this island is con- 

 spicuously red with this rock, but the north shore appears of the usual color. This rock is embraced between 

 two or three narrow basaltic dikes running east and west. As the dikes crumble by reason of their more close 

 jointage the surfaces of this red rock stand out to view. The island next further northwest (Little Brick island) 

 appears reddish in the same way OB the south side. 



Ref. Annual Report, ix, pages 67, 68 (rock No. 1845); Annual Report, xiii, pages 100 (No. 164), 103; Bulletin 

 viii, page xxxiii; BAYLEY, Bulletin cix, U. S. Geol. Survey. 



