TRANSACTIONS OP THE SECTIONS. 43 



this barrier, which has been subsequently widened by the force of waters, acting under 

 the pressure of strong west and north-west winds, at an epoch when its water-line 

 rested at one of its higher levels ; so that, at this time, Isle Royal, Beaver and Castle 

 islands, and the elevated and precipitous range of Keweena Point, all of which con- 

 sist of members of the trap rock, are the only existing monuments of this ancient 

 dyke. The heavy beds of trap boulders which lie east of this point, and reach, in 

 blocks of large magnitude, to St. Mary's Falls and the northern shores of Lake 

 Huron, strongly denote the probability of such action. 



The most extensive effects of the existing energies of this lake are witnessed upon 

 its grauwacke and sandstones, which have been broken up, comminuted into fine 

 sand, and piled up in elevated ridges, or spread out over wide plains along its south- 

 ern margin. A coast of winding bays and headlands, which measures, by a reduced 

 computation, 450 miles upon this single section, may be conjectured to have en- 

 countered heavy inroads from currents forced across the lake by north winds, or 

 acting diagonally from the north-east or north-west. By far the most extensive field 

 of this action occurs between the easterly termination of the primary series of rocks 

 at or near Granite Point, and their re-appearaace in the elevated mountainous 

 range of Gros Cape at the head of St. Mary's Straits. The vast sand dunes on this 

 section, to which the French Couriers du Bois early applied the name of Les Grandes 

 Sables, constitute a most unique and picturesque object. Their perfect aridity and 

 height above the lake, which has been computed at 300 feet, and the general paral- 

 lelism of the tops of the series of hills, strongly fix attention. These sandy eleva- 

 tions are found, however, to rest on beds of clay, loam, and gravel, of a compact 

 structure, which are only buried beneath a deep coating or upper stratum of 

 loose yellow 6and, manifestly washed up by the waves and driven landward by 

 the winds. Tempests of sand are thus formed, which spread inland, bury or kill 

 the tallest trees, and carry destruction and desolation in their track. Such is also 

 the lake action upon districts of the coasts of Huron and Michigan, the two next 

 in the descending series of the lakes. Dunes are at first formed, which spread inland, 

 carrying sterility over thousands of acres of land formerly fertile and well-wooded ; 

 and the tendency of this peculiar formation is constantly to extend its limits and 

 arrest, as with the hand of death, the progress of vegetation. Another effect of these 

 sand-rocks is to form ponds and lagoons at the temporary or fixed points of their 

 terminus on the good land, and thus to destroy and render unfit for the use of man 

 other large belts of country ; besides which, these arrested waters are the prolific 

 sources of noxious vapours, generating extensive disease in their vicinity. Evidence 

 of the comparatively recent era of this atmospheric formation is seen in the pro- 

 strated and buried trees, freshwater shells and other organic substances, in a perfectly 

 unaltered state, which are in some localities noticed in digging at great depths, and 

 sometimes exposed by recent irruptions of the waves. 



Another arenaceous formation clue to lake action cannot be mistaken in exami- 

 ning these shores, but is of an earlier age, when these lakes stood at a more elevated 

 level, and discharged over their eventual outlet at Niagara a greater volume of water. 

 " I allude to large belts and tracts of sandy plains bordering other sections of the lakes, 

 and bearing a light growth of pines, poplars and birch, which but imperfectly conceal 

 their comparatively recent origin. On penetrating these plains ridges of sand occur, 

 lying in wind-rows, as if recently formed by the winds and waves. The trees are of 

 small diameter, and the wood of a loose and bad texture. The pent-up water be- 

 tween these ridges nourishes an aquatic vegetation, and constitutes a favourite retreat 

 for the small-furred animals. The whole aspect of these plains denotes them to be 

 of freshwater origin, and forces the conclusion, that they must have emerged, at no 

 ancient period, from the watery dominion of the lakes. That these lakes stood, in 

 bygone days, at a higher altitude, that they had several epochs of depression, and 

 were thus endowed with far greater powers of geological action, is clearly denoted by 

 the existing water-lines on the mural faces of these rocks, and by the ancient water- 

 edges of pebble beds and lake exuvise found at elevated points in the interior." 



The power of attrition possessed by these lakes, at this day, is so complete upon 

 the sandstone series, as to allow full scope to the principle of gravitation in the re- 

 arrangement of the comminuted and up-heaved materials. Large portions of the 

 magnetic oxide of iron exist in the northern sandstones. As these surcharged strata 



