838 THE GEOLOGY OF MINNESOTA. 



[Granite. Diabase . 



Meg. A coarse, compact, greenish quartzyte or conglomerate with a dark, fine- 

 grained cement in which are numerous rounded fragments of quartz, feldspar, 

 quartzyte and " red rock." No section. 



Age. Potsdam. u. s. o. 



No. 2078. GRANITE. (Porphyritic.) 



St. Cloud : used for the new water-power dam at Minneapolis; also for monuments. 

 Ref. Annual Report, xxiv, page 39. 



Meg. Coarse, often reddish, usually gray. 



Mic. The large crystals are of orthnclaxe, but are closely ingrown with sec- 

 ondary albite(?) making a microperthite. The extinction angle of the former, on a 

 section cut perpendicular to the axis n g , which is perpendicular to the cleavage 010, 

 is 5 ; but on the associated albite measured on the same cleavage is 10. In the same 

 section these albite intergrowths are narrow, interrupted and yet substantially par- 

 allel with themselves, making an angle with the same cleavage of about 70. Other 

 triclinic feldspars are probably of oltgodase, but there is no favorable section for 

 specifying them in the slide. In one of the orthoclases is a minute vermicular micro- 

 pegmatyte of quartz. At the same time quartz in large grains constitutes an impor- 

 tant portion of the rock. The dark element is mostly biotite, which has numerous 

 inclusions of apatite and a few of magnetite, and others of zircon, about which are 

 formed conspicuous dark aureoles. Hornblende, of which there is but little in the 

 rock, is in some degree converted to chlorite. Within the feldspars are distributed 

 irregularly multitudes of minute scales of Muscovite. One of the feldspars presents 

 a coarse, irregular structure characteristic of microcline, but it is cut quite obliquely. 

 Within this crystal is also seen the same albitic intergrowth, constituting the niicrn- 

 cline microperthite, of Brogger. One section. 



Age. Archean (granite). N. H. w. 



No. 2090. DIABASE. 



Pair sample of the "gabbro" at Short Line park, near Thomson, cut by a dike sixty feet wide. 

 Ref. Annual Report, xxiv, page 40. 



Meg. A rather coarse-grained, diabasic rock, much altered and now composed 

 largely of hornblende which in places is in areas which give a lustre-mottled aspect 

 to the specimen. 



Mic. The rock has a distinctly ophitic structure with uralitized augite and 

 small feldspars. It also seems to have contained originally a considerable amount 

 of zirkelyte, and at the same time some of the feldspars are so decayed that they 

 consist of a confused lot of crystalliths, magnetite, epidote, mica( ?), and probably 

 others, which gives them a resemblance to the zirkelyte as now de vitrified. 



Age. Beaver Bay diabase. Cabotian. 



Remark. It is supposed that the alteration seen in this rock took place after 

 consolidation, but prior to cooling. N. H. w. 



