526 THE MARQUETTE IRON BEARING DISTRICT. 



The topography is as simple as the structure. The Michigamme 

 River, on entering the synehne about 1 inik^ south of Lake Michigamme, 

 flows through the trough nearly to its southeastern end, mainly over the 

 upper members of the bedded series. The river valley substantially 

 coincides with the bedded rocks. East and west it is flanked by Archean 

 uplands, consisting of rounded granite knobs of characteristic glacial and 

 disintegration forms, often bare or covered with a thin drift mantle. In the 

 immediate neighborhood of the southeastern termination of the trough 

 the liver first swings to the east into the eastern granite wall, and then 

 returns to the southwest, occupying a large part of the interior of the 

 trough in the structurally determined expansion of Smiths Bay, and finally 

 leaving it on the western side, about three-quarters of a mile northwest of 

 its southeastern end. Within the general topographic depression bounded 

 by the Ai'cheau areas the bedded rocks and the greenstone intrusives 

 within them occasionally form considerable elevations, none of which, 

 except Republic Mountain itself, reaches the average height of the granite 

 uplands. 



The rocks of the Republic area consist of (1) granites, gneisses, and 

 crystalline schists, which form the basement upon which the iron-bearing 

 series were laid down; (2) qiiartzites, mica-schists, and ferruginous schists, 

 of both Lower and Upper Marquette age; and (3) later igneous intrusives. 



SECTION I.— THE AKCHEAIiT. 



The granites, gneisses, and crystalline schists here constitute the unclas- 

 sified Ai-chean. These rocks have been studied only incidentally near their 

 contacts with the iron-bearing series, and chiefly from the point of view of 

 their structural relations with the latter. It appears that of the three kinds 

 of rocks into which the Archean may be divided the granites are by far 

 the most common* These are usually normal granular rocks, made up of 

 orthoclase and microcline, plagioclase, quartz, light and dark colored mica, 

 and often hornblende, with the usual accessory minerals. Often the ortho- 

 clase is present in hlrge porphyritic Carlsbad twins, which sometimes attain 

 a length of 2 inches. This coarse-grained granite is the prevailing type at 

 Republic. It weathers light-gray or white, sometimes Avith a marked red 

 tinge. The constituent minerals show no parallel aiTangement. 



