In descending the Mississippi below the Pakágama, the first stratum of rock, which rises through the delta of the river, occurs between the mouth of the Nokasippi and Elm Rivers, below the influx of the Great De Corbeau. This rock, which is greenstone trap, rises conspicuously in the bed of the stream, in a rocky isle seated in the rapid called--I know not with what propriety--the BIG FALLS, or _Grande Chute_. The precipitous and angular falls of this striking object decide that the bed of the stream is at this point on the igneous granitical and greenstone series. This formation is seen at a few points above the water, until we pass some bold and striking eminences of shining and highly crystalline hornblendic sienite, which rises in the elevation called by us Peace Rock, on the left bank, near the Osaukis Rapids. This rock lies directly opposite to the principal encampment on the 27th of July, which was on an elevated prairie on the west bank. To this point a delegation of Sioux had ascended on an embassy of peace from Fort Snelling to the Chippewas, having affixed on a pole what the exploring party called a bark letter, the ideas being represented symbolically by a species of picture writing, or hieroglyphics. In allusion to this embassy, this locality was called the Peace Rock. This rock is sienite. It is highly crystalline, and extends several miles. Its position must be, from the best accounts, in north latitude about 44° 30´. From this point to Rum River, a distance of seventy miles, no other point of the intrusion of this formation above the prairie soil was observed.