Geology of Lake Superior. 59 



It is more commonly red, and is composed of the debris of gra- 

 nitoid rocks, in nearly horizontal strata, except near intrusive rocks, 

 when it rises to a high angle, hardens, and even passes into true 

 jasper, porphyry, gneiss, or quartzite. There is reason to think 

 that this sandstone is interleaved with trap. (A Landscape was ex- 

 hibited of the Sandstone Rocks, south shore.) 



The conglomerate is of the same age with much of the sandstone ; 

 and is almost invariably placed between it and the trap. 



The conglomerates of Keweenaw and Isle Royale, consist of 

 rounded boulders of trap, with a few jaspers, cemented by red iron 

 sand ; but those of Memince and Nipigon contain also granites, 

 quartzites, and sandstones ; thus indicating a difference of age. 



3. Igneous Rocks. — Granite everywhere forms the nucleus of an 

 anticlinal axis, in two parallel lines running E. and W. on the south- 

 east side of the lake, flanked by metamorphic and sedimentary rocks. 

 Both it and syenite are plentiful. 



Trap Rocks. — The ancient lavas of the lake are in very large 

 quantities, and are well displayed. They are the great depositories 

 of copper. For convenience sake, they may be divided into three 

 principal forms. 



1st. The highly crystalline mountain masses, — sometimes anti- 

 clinal and syenitic. 

 2d. The bedded trap, at various angles of inclination. 

 3d. Dikes intersecting igneous and metamorphic rocks. 



They are all portions of one long series of volcanic operations. 



Trap creates the great headland of Keweenaw, with its lines of 

 stair-like cliffs and hills. (It was shewn in a large diagram and 

 described as typical of the trap of the whole lake.) The trap of 

 Keweenaw is met with in three contiguous and parallel belts, going 

 WSW., and separated by bands of conglomerate, sometimes very 

 thin, often numerous, and prolonged sometimes for 40 or 50 miles. 

 These three belts have been named the outer, northern, and southern ; 

 the last being highly crystalline, or syenitic, and abounding in chlorite. 

 It is an anticlinal to the rocks on both sides. The other two belts 

 are bedded traps, and with their interleaved conglomerates dip 

 northerly. They all coalesce at Portage Lake, and after proceeding 

 to Montreal River, 130 miles in the. whole, soon after disappear 

 under horizontal sandstone westwards. 



The north belt is the most metalliferous ; and contains the cele- 

 brated Cliff and other rich mines. In the Keweenaw district it is 

 the cross vein which yields the native copper — either in sheets and 

 blocks or mixed in with the usual crystallizations, such as datholite, 

 prehnite, stilbite, quartz, &c. 



On the Ontonagon River the metalliferous veins run with the strike. 

 The copper is pure, and has interspersed through its substance scales 

 of pure silver ; but without chemical union. 



