Fig. 82. River issuing from a Swiss Glacier.

epoch of thaw and floods, the power of the waters was sufficient to move pebbles of the size of a pigeon's egg. I have observed in Middle and Southern Alabama multitudes of quartzose and other hard pebbles that could not have been derived from any source nearer than the spurs of the Appalachian ridges in the northern portion of the state. One noteworthy locality is along the gorge of the Black Warrior River, at Tuscaloosa, where Sir Charles Lyell, when on his second tour through the United States, mistook them for the "shingle" of the Cretaceous system. This system produces no such pebbles in Alabama. Another locality worthy of mention is at Jackson, on the Tombigbee River, in the southern part of the state, where they constitute a bank a hundred feet in depth. In short, these pebbles may be traced all the way to the Gulf of Mexico; but their normal position is always in the deeper portions of the superficial accumulations. When the pow

er of the transporting currents grew feebler, they bore forward only the finer sands and aluminous sediments which repose generally upon the surface of the Southern States.

The rushing torrents born of the dissolving glacier busied themselves also with the work of excavation. Many an existing valley and river course was determined by the active erosions of this epoch. Many a cut through the rocky ribs of mountains had now to be executed to make way for the escape of imprisoned waters. Many a broad and rock-floored valley became filled, and converted into an alluvial plain, by the rubbish which the torrent deposited in its quieter mood. Many a basin was now scooped out which, in the next epoch, became a lake of standing water. The basins of all the larger lakes that have been excavated by erosive action conform in their longitudinal extent to the strike of the underlying formations. A line running through the centres of the great lakes from Chicago to Oswego, runs approximately along the winding strike of the formations of a certain age. This line shows the configuration of the shore of the continent when those formations were accumulating. It is worthy of particular note that the shore-line was always substantially parallel to the axis of these fresh waters during all paleozoic time. In the Lower Silurian it lay to the north of these waters. During the Devonian it was to the south of the waters.

During the

Upper Silurian it was to the south in the eastern region, and to the north (or northwest) in the western region. We may here seize upon a key to carry with us, and unlock at any time the geological map of the country before the mind's eye (compare Fig. 58). Every one locates instantly and definitely the Niagara Falls and Niagara River. The Niagara limestone was named from the falls, and its outcropping belt trends east and west at that point. This is the great limestone mass of the Upper Silurian. As in

New York, the growth of the continent was toward the south, the rocks of the Lower Silurian must lie to the north of Niagara Falls, and the rocks of the Devonian to the south. From either of these regions trace a line parallel with the axis of the lake waters-omitting Lake Superior —and we have the geographical boundary of a system of rocks, or one of the shore-lines of the ancient continent.

It is a curious fact that the great lakes were excavated along the outcrops of the formations instead of across them. It is not an unaccountable fact, for the lines of least resistance must have run along the trends of the most friable strata. Lake Michigan is scooped out from Devonian formations; and the same is true of Lake Erie. Lake Ontario is excavated in Lower Silurian strata; and the same is true of Georgian Bay, Green Bay, and the Wisconsin lakes farther south-Winnebago, Horicon, and Albion. The basin. of Lake Huron is underlaid by Devonian and Upper Silurian rocks. It seems to be two basins coalesced; and but for the peninsula of Niagara limestone separating it from Georgian Bay, it would be three basins blended in one. Lake Champlain also conforms to the trend of Lower Silurian strata, but the small meridional lakes of Central and Western New York are plowed across the formations. They are a kind of inland fiords, worked out perhaps rather by the action of the glacier than by that of the floods which followed.

The influence of these vast inland accumulations of fresh water upon the comfort and happiness of man is strikingly beneficent and providential. They serve as equalizers of summer and winter temperatures. In winter they may be regarded as vast reservoirs of warmth-great natural stoves or heaters, which continue to impart their warmth. to the frigid winds that move over them, and thus transfer their influence to the contiguous lands. This is a provision

which, till very recently, has been overlooked. It has been well understood that the Atlantic ameliorates the climate of Western Europe, and the Pacific that of Western America. I have had occasion to ascertain that a similar influence is exerted by the great lakes, and to an extent which is far more than proportional to their volume, as compared with one of the oceans. I have investigated the climate and productions of the belt along the eastern side of Lake Michigan, from St. Joseph to Mackinac, and especially in the "Grand Traverse Region," where the bays penetrate far inland, and thus augment the climatic influence of the water. In the Grand Traverse region the thermometer never sinks more than fourteen degrees below zero, and hence none of the more delicate fruit-trees ever suffer injury from the severity of the winter. Autumnal frosts are delayed till late in October, and hence the season is sufficiently long for the ripening of peaches and grapes. Snow falls in November or December, before severe freezing weather arrives, and hence the ground is never frozen, and tender roots stand out through the winter. In extreme winter weather the eastern shore of the lake is from fifteen to twenty degrees warmer than the immediate western shore. But the western shore, as that industrious physicist and archæologist, Dr. I. A. Lapham, has shown, is sensibly milder than the interior of Wisconsin, so that the ameliorating influence of the lake upon the climate of Michigan becomes strikingly manifest. No Northern state can compete with Michigan in the production of fruits. This fact, to a great extent, is owing to its environment by the great lakes. The western slope of the state is most favorably circumstanced in this respect.

Lake Michigan is a body of water three hundred miles. long, sixty miles wide, and eight hundred feet deep. The bottom is warmed by the internal fires of the earth. The

water stands at least fifteen degrees above the mean temperature of the year in the same latitude. But, even without this warming influence, the mean of the climate is considerably above the freezing point, and the cold of winter does not suffice to depress so large a body of water to thirty-two degrees. The lake, therefore, never sinks below thirty-eight or forty degrees. The bitter westerly winds, consequently, in sweeping across the lake, experience a material softening before they strike the Michigan side. It is worthy of note that, throughout the Northwest, the severest winter winds come from the west and southwest. It is for this reason that the eastern shores of the great lakes are more benefited than the western. As the bitterest winds of all are from the southwest, it follows that a situation which, like the Grand Traverse region, can receive the winds that have traveled the longest distance over the lake, will be best protected from the frosts of winter.

It is probable that the Canadian region, along the eastern shore of Lake Huron, enjoys a winter climate similarly exempt from destructive extremes. The influence of these lakes is sensibly felt even along their northern shores. The region south of Lake Ontario has long been celebrated for its fruits, while the southern shore of Lake Erie has been proven one of the best grape-producing districts of the world.

Such, then, are some of the beneficent results of an incident of the epoch of the dissolution of the glacier. The ice was rapidly melted; torrents sprang into existence, and scooped out lake basins; these became filled with waters which, besides subserving the interests of navigation, exert, perhaps, a more beneficial influence in ameliorating the condition of man in the centre of the continent.

A different ordeal still awaited the destined dwelling