The convulsions which closed the Potsdam period protruded through this formation enormous vertical walls of

ed between the lines of outburst deep valleys, which have become the bed of the lake. The sandstone which plunges beneath the water's surface on the northwest side of Kewenaw Point reappears on Isle Royale, which was formed, like Kewenaw Point, by an outburst of dolerite (Fig. 32).

Fig. 31. Section across Lake Superior, along the line XX, Fig. 30. a, a. The water level. b. Trap outburst north of the lake. c. Trap outburst forming Isle Royale. d. Trap outburst forming Kewenaw Point. é. Lake Superior sandstone and conglomerate. f. Eözoic and other rocks underlying the sand

stone.

From the north side of Isle Royale the sandstone glides under the water again, and reappears upon the northern shore of the lake. The basin of the lake is therefore a geological valley-a "synclinal" valley-formed by the igneous eruptions upon the northern and southern shores. Its origin, as will be seen, is entirely different from the origin of any of the other lake basins of the chain.

The escape of the molten rocks of the region fused out the copper and silver, which were disseminated through the neighboring strata, and accumulated them in masses of great commercial importance. An enormous dike of

dolerite, three or four miles in width, runs like a backbone from the head of Kewenaw Point, southwest and west, to the neighborhood of Ontonagon (see Fig. 30). Upon this the copper mines of the region are located. In each direction from this backbone slopes the sandstone which was upturned by it (Fig. 31). Directly across this adamantine ridge passes a stupendous cleft, which has been filled with water from the lake. Thus has been formed Portage Lake, a narrow, winding body of water, which vessels navigate. from the east to within two or three miles of Lake Superior on the west. Commerce has undertaken to complete the work begun by Nature, and soon Kewenaw Point will be an island.

Toward the eastern extremity of Lake Superior the formation reposes in nearly horizontal beds, and the erosion. of the lake along the southern shore has carved out bold escarpments which arrest the attention of every traveler. These have been named the "Pictured Rocks" (Fig. 32), from the diversified colorings of the various portions of the sandstone mass. It is a dangerous coast, and no species. of craft ventures within inspection distance except in calm and settled weather. The high bold wall looks sheer across the lake, and the storm-wind rolls tremendous surges against its stern, defiant face. They have excavated caverns which a canoe may traverse, and in which the imprisoned billows howl with the resonance of a Nahant purgatory. They have carved out mimic architecture and spread a mimic sail. In times long gone by they have flanked a stubborn bluff, and cut it off from the main land by a military "ditch" of the most successful kind, since it is a mile wide, and is navigated by the largest vessels. The isolated piece is known as Grand Island.

"The range of cliffs to which the name of Pictured Rocks has been given," say Foster and Whitney," may be regard

Fig. 32. The Miners' Castle--"Pictured Rocks," Lake Superior.

ed as among the most striking and beautiful features of the scenery of the Northwest, and are well worthy of the attention of the artist, of the lover of the grand and beautiful, and of the observer of geological phenomena.”*

The first display of architectural mimicry witnessed in coasting eastward is a salient mass of sandstone known as the "Miner's Castle," presenting the turreted elevation and arched and massive doorway of some ancient feudal seat. The height of the doorway is about seventy feet, while the tops of the turrets are one hundred and forty feet above the lake (Fig. 32).

About five miles farther eastward the cliffs attain an elevation of about one hundred and seventy-three feet, presenting a series of sinuosities or scollops hewn out by the action of the waves. One of the grandest and most regular of these was named "The Amphitheatre" by Foster and Whitney. Still farther eastward this scolloped contour graduates into extravagant carvings, which have wrought the mural wall into wierd Titanic mimicry of architectural forms.. Vast tablets from the upper courses of the wall, sapped by the agency of eroding waves, have tumbled down and strewn the beach in places with fragments which lead the traveler to believe he is clambering among the ruins of gigantic temples shattered by an earthquake shudder. A group of these fallen fragments presents a striking similitude to the jib and mainsail of a sloop full spread,

*Report on the Geology of the Lake Superior Land District, part ii.; p. 124 (1857). These authors have given the fullest and most exact account of the Pictured Rocks that has yet been published. Schoolcraft, at an early period, undertook to describe this range of cliffs and illustrate the scenery, but with very poor success. Harper's Magazine, vol. xxxiv., p. 681 (May, 1867), contains a lively and interesting paper on the "Pictured Rocks," embodying several good illustrations. Some of the following views are from photographs by Watson, taken on an excursion by a party from the University of Michigan in 1868, under the leadership of Dr. A. E. Foote.