Fig. 81. Sketch of Glacier Furrows and Scratches at Stony Pt., Lake Erie, Mich. a, a. Deep water line. b, b, Border of the bank of earthy materials. c, c. Deep parallel grooves 4% feet apart and 25 feet long, bearing N. 60° E. d. A set of grooves and scratches bearing N. 60° W. e. A natural bridge.

A result of this wide-spread scouring and grinding of the rocks was the accumulation of vast quantities of detritus. From this source comes a large proportion of the pebbles, sand, and clay which every where underlie the surface-soil, and separate it from the bed-rock-an essential and beneficent provision, as every one knows who has observed the destructive effects of ordinary droughts upon thin soils resting on a rocky basis. Another effect of the great glacier was the destruction of all vegetation over the areas which it invaded. From season to season, and from year to year, the mighty mass marched irresistibly forward,

mowing down the forests, crushing tree-trunks, or burying them, with the rubbish of the rocks, from ten to sixty feet beneath the surface. Such buried tree-trunks have thus lain to the present day, and we frequently encounter them in deep excavations for wells, though my friend Professor Lesquereux has strangely asserted the contrary. With other relics of the vegetation of the ancient world were necessarily buried the seeds and fruits of the species then in existence, a fact of which I shall find the use hereafter, in speaking of prairies.

The great glacier moved onward, unheeding equally rocky knob, and swelling hill, and river gorge. I have stated that from the close of the Carboniferous Age the Northern States were dry land. Rains fell, as now, upon the surface, and nourished the vegetation which had found a foothold. The surplus waters gathered themselves, as now, into streamlets large and small, and these, on their way to the sea, wore river-channels in the surface rocks. Across these rivers, across these gorges, the great glacier strode, ignorant of the obstacles to its movement. It bridged Niagara River, it bridged Long Island Sound, and bathed itself in the mild waters of the ocean beyond. It obliterated river-channels, and dug out new ones. plowed anew the country marked off by the feebler agencies of the preceding epoch. It made a tabula rasa, and outlined after a different pattern the topographical and hydrographical features of the Northern States. Many an ancient river-channel has been brought to light by railroad excavations, and more especially by the borings for petroleum that have taken place within the last few years. In many instances the general rocky structure of a region has determined the location of the streams through the same valleys as before the work of the glacier; but even here we find the position slightly varied, and in nearly all cases

It

the present channel is a narrow and shallow one, excavated through the surface of the loose materials which fill the more extensive ancient channel. In Ohio and Indiana these buried river-beds are of frequent occurrence. The ancient gorge of the Niagara River was filled by the obliterating agency of this continental glacier. For ages and ages the river had patiently labored upon this excavation, as it has since done upon the existing one; but the glacier came with its cubic miles of rubbish, and wiped out. the trifling furrow, leaving the surface comparatively level, and making it necessary for the river to begin anew its work when the invading glacier had disappeared. The excavation of lake basins is sometimes attributed to this agency, but these may have been partly the result of subsequent aqueous action. It was probably the force which dug the shores of northern seas into their numerous deep and narrow fiords, as can be seen upon the coast of Maine, and the European and Asiatic shores of the Arctic Ocean. It bore southward, over distances of twenty, fifty, and even five hundred miles, fragments of Northern rocks, some of which are of enormous magnitude. One in Bradford, Massachusetts, is thirty feet each way, and weighs not less than four and a half millions of pounds. A boulder of jaspery conglomerate, weighing about seven tons, was transported three quarters of a mile by the class of 1862, and mounted upon the campus of the University of Michigan, an imperishable, monument to their memory and their enterprise. The native home of this huge mass is the northern shore of Lake Huron, where the formation is found in place, and where I have seen detached and rounded masses weighing probably a hundred tons. These fragments have thus been transported over lakes, sounds, and seas. Masses of native copper from Lake Superior are strewn over Wisconsin and Lower Michigan, and have wandered even into Ohio and

Indiana; while pebbles of quartz, gneiss, granite, dolerite, and other rocks from the same regions constitute a large proportion of the soil of these states. The streets of Cincinnati are paved with stones which were quarried by the hand of Nature in the region of the upper lakes.

Professor Agassiz, to whom we are indebted for the full exposition and application of the glacial theory, thinks he discovers abundant evidences of the former action of glaciers in Brazil; but the presence of rocky débris, and even of rounded pebbles that can not be attributed to shore action, is not enough to establish glacial agency, especially while in the United States we do not recognize it south of the Ohio River. On the contrary, Professor Whitney has. recently asserted that the proofs of glacial action are entirely wanting in California, and for some distance northward. The copious accumulations of unsolidified surface materials are attributed to the slow disintegration of the rocks under atmospheric agencies.

Glaciers of almost continental extent still exist on the shores of Greenland, and, cover the Antarctic land discovered by the United States Exploring Expedition; also Wrangell Land, very recently discovered by Captain Long in the Arctic Ocean. Perennial ice binds the soil of Northern Siberia, and, as is well known, preserved for many centuries the carcasses of hairy elephants incased in it. There is little difficulty in believing that these high-latitude icefields are merely the remnants of glaciers which once extended many degrees farther toward the south.

CHAPTER XX.

LABORS OF THE ICE-BORN TORRENTS, AND THE OCEAN

THE

BURIAL.

HE manacles of ice were loosened by the genius of a geological spring-time. Next in the order of vicissitudes was a grand continental subsidence. Vast areas of Northern America, that had been raised to the altitude of perpetual snow, were gradually lowered to the ocean's level. Again the interchange of equatorial and polar temperatures was effected by the moving sea-currents, and the climate of summer smiled over the desolate empire of frost. The rocky glacier yielded to the touch of warmth, and a myriad streams leaped from the bosom of the snow (Fig. 82). Each ice-cold rill united with its fellow, and a deluge of waters set out on their journey to the sea. They wound their way across the future states of Kentucky, Tennessee, and Alabama, to the Gulf. They bore forward a freight of sediments selected from the rubbish bequeathed by the dying glacier, and strewed it over the states that had not been visited by the beneficent action of the ice. Thus the Gulf States and the middle-latitude states shared with the northern regions the materials prepared to serve as the basis of soils in the coming age of thought and industry. These myriad streamlets were, however, unable to bear forward the boulders which had been carried by the ice to the borders of the Southern States. And hence it is that, south of the Ohio, "cobble-stones" are sought in vain. The soil and subsoil possess a degree of fineness and homogeneousness not characteristic of the surface deposits of the Northern States. In the earlier portion of the