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. [226] I would also refer, for subsequent information, to my report of the 1st of October, 1822, made in compliance to a resolution of the Senate, and printed in the Executive Documents of that year, No. 365, 17th Congress, 2d session. 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. LA POINTE CHEGOIMEGON.--A sketch of these islands, as given in the Narrative, denotes that their number is greatly underrated, and will serve to show the configuration of a very marked part of the Superior coast. It must, hereafter, become one of the principal harbors and anchoring-ground for vessels of the lake.