



390 THE GEOLOGY OF MINNESOTA. 



, [Gabbro and Granite. 



Meg. A very fine-grained, greenish-gray, slaty rock. 



Mic. . The section shows none of the large grains ot quartz &nd feldspar common 

 in the associated graywackes. It is composed of a greenish mass which under a high 

 power is seen to be made up essentially of minute flakes of chlorite and a micaceous 

 mineral (probably sericite), and minute grains of quartz and feldspar. A few grains 

 of epidote are also seen, and there are specks which appear yellow in reflected light 

 which are perhaps composed of very minutely granular e/w/ote. 



One section. 



Aye. Keewatin(?) 



Remark. In composition this rock is essentially the same as the greenish 

 "groundmass" of the graywackes of this locality. It simply represents the finer 

 material of these rocks and it lacks the coarser grains of quartz and feldspar which 

 are so characteristic a feature of the associated graywacke beds. 



This rock is the last of the graywackes from the St. Louis river, and in this 

 connection it is necessary to refer to the description of Dr. J. H. Kloos of rocks from 

 the vicinity of Thomson (Annual Report, xix, pages 109-113). He described in detail 

 two rocks, which he termed horn-slate and roofing-slate. The former evidently was 

 a medium or fine-grained graywacke (compare Nos. 473, 508, 509, and others), while 

 the latter seems to have been one of the black argillaceous slates which vary to 

 graywacke slates like No. 510. u. s. G. 



The doubtful assignment of the rocks from Thomson northward to Cloquet to 

 the Keewatin, is designed to express a slight preference for that rather than for 

 Animikie. The more probably Animikie part of this series lies further south, 

 appearing at the lower falls of the St. Louis, and at the cuts made by the Northern 

 Pacific railroad. The question of the age of the Thomson rocks is considered in vol. 

 iv, page 551. N. H. w. 



No. 511. GABBRO AND GRANITE. (Contact.) 



Duluth. At a point in the bed of Miller's creek, near the quarter section line of sec. 32, T. 50-14, 424 feet 

 above lake Superior. This rock extends indefinite!}' up stream. 

 Ref. Annual Report, x, pages 34, 35; Bulletin ii, pago i'l. 



Meg. The specimen consists of two rocks, a reddish granite and a gray gabbro. 



Mir. The red rock has much quartz, both in isolated grains and in finer 

 pegmatitic growths in the feldspars. The feldspar is stained with hematite, a fact 

 which causes the general redness of the rock. It cannot be specifically determined, 

 but is probably largely of orthoclase, yet occasionally along with some evident 

 plagioclase a trace of very fine albite twinning can be seen, indicating, in this situation, 

 that some of the feldspar is anorthoclase. No other mineral is abundant. There is 

 some honi/i/ciH/i', a little biotite and chlorite, and an occasional 



