Volcanic action, to which this disturbance in its westerly bearings may be attributed, appears to have thrown up the trap-rocks of the Pic, of the Porcupine chain, of the Isle Royal group, and other trap islands, and the long peninsula of Keweena. This system of forces appears to have spent itself from the northeast to the southwest. The shocks brought with them the elements of the copper and other metallic bodies which characterize the trap-rock. They exhausted their power, on the American side, west of the granitic tract of Chocolate and Dead Rivers, and the Totosh and Cradle-Top Mountains. The most violent disturbance took place at the west of the Keweena Peninsula, and thence it was propagated in the direction of the higher Ontonagon, the Iron, and the Montreal rivers.
This disturbance of the level of the sandstone produced undulations, which are observable on the St. Mary's, where the variation from a level is not more than eight or ten degrees. They left portions of it—as between Isle au Train and the Firesteel River—undisturbed; and they threw other portions of it—as between Iron and Montreal rivers—almost completely on their edges.
The entire north shore from Gargontwa to the old Grand Portage, inclusive of the Michepicotin and Pic regions, cannot be particularly alluded to, as that part of the coast was not visited; but the accounts of observers represent it as consisting of trap-rocks. Without the application of such forces, it appears impossible to understand the geology of this lake, or to account for the sectional and disturbed formations.
The lake itself, whose depth is great, and which has an extreme length of about 500 miles, by an extreme width of some 180, is endowed with powerful means of existing elemental action. This consists almost entirely of the force of its winds and long, sweeping waves. Its bottom may, in this light, be looked upon as an immense mortar or triturating apparatus, in which its sandstones, trap-boulders, and pebbles are driven about and comminuted. This power has greatly changed its configuration, and the process of these mutations is daily going on.
It is only by such a power of geological action that we can account for the powerful demolitions and inroads which it has made upon some parts of its southern borders. The coasts of the Pictured Rocks, which have a prominent development of about 12 to 15 miles, consist in horizontal strata of coarse gray sandstone, of little cohering power. The effect of waves beating upon rocks is to communicate a curved line. This has operated to excavate numerous and extensive caves into the coast. These, after reaching hundreds of feet, have in some cases united. The effect is to isolate portions of the coast, and to leave it in fearful pinnacles, having many of the architectural characters of Gothic or Doric ruins.
The portion of coast immediately west of Grand Marrais is scarcely less unique. It denotes the effect of the prostrating power of the lake in another way. The sandstone of parts of the coast, ground down into yellow sand by this vast machinery, is lifted up by the winds as soon as it reaches the point of dryness, and heaped up into vast dunes. Standing trees are buried in these tempests of sand, and its effect is, for about nine miles along the coast, to present, at an elevation of several hundred feet, a scene of arid desolation, which can only be equalled by the Arabic deserts.
A dyke of trap seems once to have extended from the north shore to Point Keweena; but, if so, it has been prostrated, and its contents—veins and deposits, silicious and metallic—scattered profusely around the shores of the lakes. A cause less general is hardly sufficient to account for the wide distribution of fragments of the copper veins and vein-stones which have so long been noticed as characters of this lake. The basal remains of this antique dyke form the peninsula of Keweena. The tempests beating against this barrier from the northwest, have ripped up terrific areas from the solid rock, and left its covering, amygdaloid and rubblestones, in fantastic patches upon the more solid parts, or constituting islands in front of them.
Structure of its Southern Coast.—The estimated distance from Sault Ste. Marie to Fond du Lac is a fraction over 500 miles. The sandstone, as it appears in the Falls of the St. Mary's, does not appear to be entirely level. It exhibits an undulation of about 8° or 10°, dipping to west-northwest. Two instances of this waved stratification of the Lake Superior sandstone deserve notice. The first terminates at the intersection of red sand rock at la Point des Grande Sables with the beginning of the horizontal strata of the Pictured Rocks. We again observe an inclination of the strata of a few degrees at Grand Island, which is moreingfish River, and appears to dip at Isle aux Trains, about twenty miles northeast. The scenery is peculiarly soft and pleasing in passing the Huron Islands, a granitic group, and directing the view, as in the sketch, to the coast and the rough granitical hills rising behind Huron Bay. The strata are level, as shown above, around the Bay of Presque Isle and Granite Point, and continue so, resting on the roots of the granitical tract of the Tötosh, or Schoolcraft, and Cradletop Mountains, and at Point aux Beignes, and Keweena Bay. This level position of the rock is preserved to the south cape of the shallow bay of the Bete Gre, on the north, at which the trap-dykes of the peninsula first begin; and so continues after passing that rugged coast of the vitreous series of that remarkable point, to and beyond Eagle River and Sandy Bay, in the approach to the portage of the Keweena.
The same horizontality is observed on the headland west of it, and upon all the points and headlands to Misery and Firesteel Rivers and the mouth of the Ontonagon. The trap-dyke of Keweena crosses this river about ten miles, in a direct line, inland.
At Iron River, we observe a stratum of compact gray grauwacke, over the hackly bed of which that river forces its way during the spring months, and stands in tanks and pools during the summer. On reaching the foot of the Porcupine Mountains, the sandstone, which is here of a dark chocolate color, with quartz pebbles of the bigness of a pigeon's egg, and organic remains of paleozoic type, is found to be tilted up into nearly a vertical position, as shown in the sketch. The grauwacke reappears, in a most striking manner, at the Falls of Presque Isle River, where the whole mass of water precipitated from the highlands drops into a vast pot-hole, a hundred feet wide and perhaps twice that depth. The whole upper series of rocks, from the Porcupine Cliffs west to the Montreal River, is a conglomerate. At the Falls of the Montreal, the river drops over the vertical edges of the red sandstone. Beyond the Bay of St. Chares, at Lapointe Chegoimigon, masses of sienitic mountains arise, which have their apex near La Riviere de Fromboise.