To prepare the mind to appreciate the account which I give of changes and developments in the physical structure of the country, it may be observed that the American continent has experienced some of the most striking mutations in its structure _at_ and _north_ of the great chain of lakes. That chain is itself rather the evidence of disruptions and upheavals of formations, which give its northern coasts, to some extent, the character of ancient--very ancient--volcanic areas of action. These lakes form--except Erie and Ontario--the general boundaries between the primitive and secondary strata. But, however striking this fact may, at particular localities, appear--such as at the Straits of St. Mary, of which the east and west shores are, geologically, of different construction--yet nothing in the grand phenomena of the whole region visited is so remarkable as the boulder stratum, which is spread, generally, from the north to the south. Some of the blocks of rock are enormous, and would seem to defy any known cause of removal from their parent beds; others are smaller, and have had their angles removed, and far the greater number of these transported boulders are quite smooth and rounded by the force of attrition. This drift stratum has been tossed and scattered from its northern latitudes over the surface of the limestones and sandstones of the south. It is mixed with the diluvial soils, in Michigan and elsewhere; but it is evident that, in its diffusion south, the heavier pieces have settled first, while comparatively minute boulders have been carried over or dropped in the plains and prairies of Ohio, Illinois, and more southerly regions. Nobody, with an eye to geology, can mistake the heavy boulder deposits which mark the southern shores of Huron, and become still more abundant on the St. Mary's, the shores of Lake Superior, and along the channels of the River St. Louis and the Upper Mississippi. Lake Superior has been the central theatre of volcanic upheavals; but they must have operated at very remote periods, for there is not only no evidence of existing volcanic fires, but the heavy debris everywhere bespeaks long intervals of quietude, and slow elementary degradation. Some of the upheavals were made after the deposition of the sandstone rocks, which are, as at the foot of the Porcupine Mountains, raised up to stand nearly vertical; while other districts of the granitic rock, as at Granite Point, had been elevated before the deposition of the sandstone rock, which is accurately adjusted to its asperities, and remains quite horizontal.