The Islands of the Twelve Apostles, or Federation Group, appear to be all based on the sienitic or trap, with overlying red sandstone; which latter again reappears on the point of the entrance into Fond du Lac Bay, and marks its southern coast, till near the entrance of the Brulé, or Misakoda River, as seen in the illustration beneath. Shores of sand then intercept its view to the entrance of the River St. Louis, and up its channel to its first rapids, about eighteen miles, where the red sandstone again appears, as the first series of the Cabotian Mountains.
Serpentine Rock.—At the nearest point north of Rivier du Mort is a headland of this rock, jutting out from the granitical formation. Lapping against it, at the mouth of the river, is a curious formation of magnesian breccia. The serpentine rock appears, in nearly every locality examined, to be highly charged with particles of chromate of iron. It may be expected to yield the usual magnesian minerals.[ [222] Its position is between the Carp River and Granite Point, in the Bay of Presque Isle, or rather Chocolate River, for that river pours into this bay by far the largest quantity of water.[ [223]
Ancient Drift-Stratum.—In the intervals between the points and headlands, where the rock formation is exposed by streams or gorges, the drift, or erratic boulder stratum, is found. Such is its position beneath the sand-dunes of the Grandes Sables, and in the elder plains and uplands, stretching with interruptions on the coast from the head of the Mary's valley to that of the St. Louis. The edge of this formation is composed of the sand and loose pebbles and boulders of the lake. Mighty as are the existing causes of action of the lake in beating down and disrupting strata of every kind, and in reproducing alluvial lands and dunes, they are weak and local when compared to the causes which have spread these ponderous boulders, and drift masses over latitudes and longitudes which appear to be limited only by the leading elevations of the continent. That oceanic torrents of water, suddenly heaped on the land, and wedged into compactness and power now unknown to it, is after all, the most plausible theory of the dispersion of this formation, and this theory avoids the necessary local one of the glacial dispersion which presupposes a very low temperature over the whole surface of the globe.
Kaugwudju.[ [224]—This imposing mass of the trap-rocks is the highest on the southern shores of Lake Superior. The following outlines of it are taken from a point on the approach to the Ontonagon River, about forty miles distant.
They rise to their apex about thirty miles west of that stream, in north lat. 46° 52´ 2´´, as observed by Captain Douglass. They are distant three hundred and fifty miles from St. Mary's. In a serene day they present a lofty outline, and were seen by us from the east, at the distance of about eighty miles. The Indians represent them to have a deep tarn, with very imposing perpendicular walls, at one of the highest points. If Lake Superior be estimated at six hundred and forty feet above the Atlantic, as my notes indicate, its peaks are higher than any estimates we have of the source of the Mississippi, and are, at least, the highest elevations on this part of the continent. The granitical tract of the St. Francis, Missouri,[ [225] and of the quartz high lands of Wachita, Arkansas, the only two known primitive elevations between the Rocky and Alleghany chains, are far less elevated.
I have now taken a rapid glance at the formations along the southern shore of the lake between St. Mary's and Fond du Lac; but have passed by some features which may be thought to merit attention.
Existing Lake Drift.—The gleaner among the rock debris of this lake has a field of labor which is not dissimilar to that of the fossilist. If he has not, so to say, to put joint to joint, to establish his conclusions, he has a mineralogical adjustment to make every way as obscure. A boulder of sienite, or a mass of sandstone, or grauwacke, may be easily referred to a contiguous rock. But when the observer meets with species which are apparently foreign to the region, he is placed in a dilemma between the toil of an impossible scrutiny and the danger of an unlicensed conjecture.
Among the more common masses which may be assigned a locality within the compass of the lake, are granites, sienites, hornblendes, greenstones, schists, traps, grauwackes, sandstones, porphyries, quartz rocks, serpentines, breccias, amygdaloids, amphiboles, and a variety of masses in which epidote and hornblende are essential constituents. With these, the coast mineralogist must associate, in place or out of place, agates, chalcedonies, carnelians, zeolite, prehnite, calcareous spar, crystalline quartz, amethystine quartz, coarse jaspers, noble serpentine, iron-sand, iron-glance, sulphate of lead, chromate of iron, native copper, carbonate of copper, and various species of pyrites. These were, at least, my principal rewards for about eighteen days' labor, in scrutinizing, at every possible point, its lengthened and varied coasts.
Cupreous Formation.—The whole region, above Grand Island at least, appears to have been the theatre of trap-dykes, and an extensive action from beneath, which brought to the surface the elements of the formation of copper veins. These have not been much explored; but, so far as observation goes, there are evidences which cannot be resisted, that the region contains this metal in various shapes and great abundance. I refer to my report of the 6th of November, 1820, for evidences of a valuable deposit of this metal in the valley of the Ontonagon River, and at other points. I found the metal in its native state at various other localities, and always under physical evidences which denoted its existence, in the geological column of the lake, in quantity. These indications were confined almost exclusively to the area intervening between the peninsula of Keweena, and La Pointe Chegoimegon, a distance of about one hundred and fifty miles. Of this district, the two extremities would make the Ontonagon Valley about the centre.[ [226] A profile of one of the detached pieces, found in the Ontonagon Valley, and forwarded to you by Mr. Van Rensselaer, is herewith given.
Vitric Boulders.—Among the debris of Lake Superior are masses of trachyte, and also small pieces of the sienitic series, in which the red feldspar has a calcined appearance, the quartz being, at the same time, converted into a perfectly vitreous texture. Similar productions, but not of the same exact character, exist on the sandy summits of the Grande Sable. These exhibit an exterior of glistening cells or orifices: it may be possible that they have been produced by fusion; but I think not. The smooth cells appear like grains of sand hurled by the winds over these bleak dunes. I have brought from that locality a single specimen of pitchstone, perfectly resinous, bleak and shining.