prairie region of Illinois. The underlying strata are nearly horizontal. Why now do they terminate so abruptly at the mountain wall which we scale to reach Bon Air? The cut margins of these mountain ribs lie exposed and protruding from base to summit of the laborious ascent. Did Nature form them originally thus? We are forced to conclude that these mountain sheets, like those under the "highland rim," once extended westward over the basin of Tennessee, and have been scooped out by some tremendous agency appointed by Nature to furnish materials for the states-then future-of Georgia, Alabama, and Mississippi. The Cumberland Table-land and the abrupt knobs about Chattanooga are not upheavals, but lines of relief. It is the valleys that have been made, and not the mountains. The mountains are landmarks-" benchmarks" the engineer might say--showing the former level of the entire region.

If we travel westward. or northward from Nashville we find the basin walled in by the "highland rim," though it is only on the east that the pile of strata rises so high as to bring us within the limits of the Coal-measures. Here, then, is one of the most stupendous examples of geological denudation. What the precise nature of the agency by which this work was done we can only conjecture. Equally uncertain is the precise date of the work. We can only say that it was performed between the close of Paleozoic Time and the present, but as to the reality and almost incalculable vastness of the work we have no room to doubt. Neither can we fail to see that such enormous excavations must have been in progress in all ages, to furnish the requisite amount of materials for formations of continental extent, and attaining a thickness of hundreds or thousands of feet.

I have already alluded to the monuments of destructive

action around the shores of the great lakes. Even mimic oceans like these, in the era of their strength, have performed labors which excite our astonishment. And that Titanic power which geology dimly pictures to us as moving in glacier-masses from parallel to parallel, riding over primeval forests, obliterating ancient river-beds, plowing out lake-basins to the depth of nine hundred feet, and crushing to powder countless cubic miles of obdurate granite and quartz-that power of which we can little more than dream, though the records of its marvelous march are scattered about on every side—a power a power which may have been summoned into exercise at more than one period in the world's history-that power whose movement was resistless as fate, and destructive as the crash of worlds, can serve at least to impress our minds with the energy of geological agencies, and the resources at Nature's hand for the scooping of lake-basins, the carving of mountain cliffs, or the scraping out of the bowels of the State of Tennessee.

Even the humble river-stream-humble by comparison, but terrific as Niagara in unwasting and untiring powerhas accomplished work at which the highest human engineering stands appalled. The Kentucky and the Cumberland, in traversing the states which they drain, have worn their channels to the depth of hundreds of feet below the general level of the country. Some of the wildest and most attractive scenery of the continent lies along the Kentucky, from the mouth of Hickman's Creek in Garrard County, to Dix River and Coger's Ferry in Mercer County. Even the smallest streams have aped the pretentious labors of the larger, and have succeeded in opening their narrower gorges through two, three, and four hundred feet of the blue limestone of the blue-grass lands of Kentucky and Tennessee.

But these all are pigmy works compared with those of

the streams which traverse the "Great American Desert." For the most vivid descriptions of the geology of this forsaken region we are indebted to Dr. Newberry, the geologist of Ives's Colorado Expedition under the general government. The surface formations are mostly of later Carboniferous and Mesozoic age, interrupted at intervals by mountain-like outbursts of volcanic origin. The region is a vast plateau stretching for hundreds of miles in either direction (Fig. 96). The floor of the plateau is a mass of horizontal strata. Far in the hazy horizon may be seen the bold wall, which rises to a more elevated table-land composed of overlying strata. These higher strata were once continuous over the surface of the lower plateau, but have been swept off by denudation. Still farther in the horizon looms up another gigantic terrace, rising to the upper plateau of the desert. The traveler journeying across this apparently monotonous and desert plain finds himself suddenly standing on the brink of a precipice. It is the wall of a deep gorge. Down into this gloomy chasm he endeavors to cast a look. It is like a vertical rent through the strata to the appalling depth of more than a mile. Far down at the bottom winds the sky-lighted stream which has executed this tremendous piece of engineering, quiet now as a lamb, but in spring-time roaring and destructive as a lion. This is the Colorado. Its immediate banks are fringed at intervals by a narrow border of grass, and these meagre grass-plots down in the rocky cleft are the occasional abode of the desert Indian. The great Black Cañon of the Colorado is a gorge with perpendicular walls. of rock three hundred miles long, and from three thousand to six thousand feet high! The lateral streams have cut similar gorges, and these almost impassable chasms constitute formidable difficulties in traversing the country. (Fig. 97). The Colorado has cut through the entire series

Fig. 96. Upper Cataract Creek, near Big Cañon, Colorado.

of formations, and sunken a thousand feet into the solid granite. The section of the rocks in the gorge shows above the granite two or three thousand feet of paleozoic sandstones, shales, and limestones, one thousand feet of subcarboniferous limestones, and twelve hundred feet of carboniferous sandstone and limestone. Higher up the stream the section extends up through the Triassic and Cretaceous systems.

What æons have rolled by while this unparalleled riverwork has been in progress! And yet this work must have been limited to the later ages, since the gorge cuts through Cretaceous strata. There was a time, during the Cenozoic ages, before yet the ridges of the Rocky Mountains had been elevated to their present altitudes, when this vast desert had just become dry land-upheaved from the recent bottom of the Cretaceous sea. Now the Colorado began to gather its forces and to irrigate the surface of the new-formed land. Now began the great cañon; but for many ages the surface features of the region were normal; and not improbably it was clothed with a soil, and watered by streams which sustained a luxuriant growth of vegetation. But man was slumbering in the voiceless future, and lazy reptiles held possession of the fair domain. Vast, then, as is the work, and vast as must have been its duration, its commencement can date back but to the end, or, at farthest, to the beginning of Cenozoic Time.

Who can tell but similar gorges have been cut in the strata of more eastern states. Here was land-permanent land-covered with vegetation, while yet the great desert was but ocean-slime. Here, too, were rivers―rivers like the Ohio and the Mississippi--with their numerous tributaries. What prevented these streams from scoring the strata to the depth of ten thousand feet? We know that during this interval the Niagara cut an ancient gorge. We