OF THE NORTHWESTERN STATES. 29 



or fifteen feet, wholly free of gravel or earth. They are from one to four miles 

 from the shore, and in shallow water. The boulders are sienite, granite, trap, 

 gneiss, &c., being the same with those which occur in the drift beds of the adjacent 

 shore. 



As the degradation of the land goes on the number of stones increases, and the 

 line becomes more conspicuous. In those northern latitudes the ice attains a thick- 

 ness of two and three feet ; thus reaching down to boulders which lie in five and 

 six feet water. In a low stage of the water, which occurs annually in the winter, 

 they may be grasped and moved at a still greater depth. 



A progress shoreward of a foot or two in a year, would, in a few centuries, 

 transport them many hundred feet. These processes now open to observation are, 

 like those on a more extended scale, of the drift era, slow but irresistible, and capa- 

 ble, after long periods, of producing great results. 



Lakes of Erosion. 



Along the north shore of Point Keweenaw, there is a series of long narrow bays, 

 the depth and contour of which are largely due to glacier excavation. Copper 

 harbor. Agate harbor. Eagle harbor, and Cat harbor are of this class. The strata 

 are alternately trap, sandstone, and conglomerate, the strike of the beds being 

 nearly east and west. Their longest axis is parallel with the strike of the recks, 

 which dip northerly. Between the lake and the waters of the harbors is a low up- 

 lift of trap, with narrow breaks, which form the entrances to still water in the 

 rear. The course of the glacier movement was here from northeast to southwest, 

 somewhat oblique to the strike of the rocks. 



Point Keweenaw is a high and narrow mountain range, the general course of 

 which is northeast and southwest, except at its eastern extremity, where it curves 

 to the east. It is at this part the harbors above named are situated. The high 

 narrow crest of the centre line of Point Keweenaw, rising six hundred to eight 

 hundred feet above the lake, modified somewhat the course of the movement. 

 Lakes Schlatter, Fanny Hooe, and Upson, on the north side of the Point, are in 

 the same elongated form east and west as the coast harbors. On the height of 

 land are Lake Manganese, Musquito lake, and Portage lake, situated in breaks of 

 the mountain range, where the mechanical efi"ects are less prominent, but they show 

 erosive action, and the change of direction due to gaps in the range. 



On the island of Isle Royal the same strata occur, with the same northeasterly 

 strike. Here there is a rapid succession of hard trap with softer sandstone and 

 conglomerate beds, their dip being to the south. This island presents the most 

 remarkable cases of erosion. Whether upon the coast or in the interior, there is 

 scarcely a square mile that does not exhibit distinct glacier action. Here also the 

 bearing of the striae very nearly coincides with the strike of the rocks. 



At the eastern extremity is a series of narrow ledges, which might be aptly com- 

 pared with the fingers of a hand, and Avhich are composed of trap ; the spaces 

 between represent numerous straits and harbors, with sandstone at the bottom. 

 There are troughs of excavation which extend southwesterly the entire length of 



