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may venture to go much farther than this, and assert that the material of thought which Nature furnishes is correlated to the thinking principle in man. When the Creator adopted an intelligent method in the ordinations of the material world, it was equivalent to a declaration of purpose to introduce an intelligent being. And when the Creator had stocked the world with the materials of thought, and had planted in it a being capable of understanding Nature, it was the obvious purpose of the Deity that Nature should be investigated, and that, by such investigations, man should become not only wiser, but more reverent, religious, and happy.

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CHAPTER XXXI.

THE TOOTH OF TIME.

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FEW words about the disintegration of the rocks. As the vital force employs itself in the demolition of the organic structures and the simultaneous repair of all the wastages, so the gigantic energies of geology have busied themselves in one age or place in demolishing the rocky fabrics consolidated with incredible labor in another age or place. The grain of sand upon the rivulet's border may have been incorporated successively into a dozen dif ferent formations, each in turn disintegrated to be inwrought in the rocky sheets of the next succeeding age.

Has the reader ever inquired whence came the materials for twenty-five miles of sedimentary strata? It is a question which geology is compelled to answer. The first and lowest great system of strata-the Laurentian-is in Canada thirty or forty thousand feet thick. This system is supposed to embrace nearly the entire globe, passing beneath the Paleozoic, Mesozoic, and Cenozoic strata, and extending, probably with greatly diminished thickness, under the beds of the existing oceans. It must have been accumulated while yet the primeval sea was wellnigh universal. This is the prevalent opinion. It is perfectly plain, however, that these vast beds of sediment must have had an origin in pre-existing rocks lying within reach of the denuding agencies of the time. How enormous a bulk of solid rocks was ground to powder to furnish material for these Laurentian strata may be imagined when the reader is reminded that the mean elevation of North America is

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but about twelve hundred feet above the level of the sea; and if the entire continent were ground to powder down to the sea-level, and distributed over an area of the ocean's bottom equal only in extent to North America, it would afford a bed of strata not one twentieth the thickness of the Laurentian system over the same region. Whence, then, the materials for so vast an accumulation of sediments? Where were the lands which must have disappeared during the Laurentian Age? Although we may not be able to indicate their location, the facts suggested serve to remind us of the gigantic scale of operations of the denuding agencies of primeval time.

Every succeeding geological age must also have had its source of supply to the contemporaneous sediments. The ever-growing continents were ever wearing down. As the increasing pressure of the accumulating oceans crowded higher the summits of the continental axes, the ceaseless demands of the insatiate sea for more sediments wore thinner and thinner their denuded scalps. It is no wonder that included fires burst forth at the summits of the highest mountains. These are the exposed points, where the earth's crust has been reduced to the greatest degree of tenuity, while the ocean's floor is the most solid portion of the globe.

The outburst along the southern shore of Lake Superior at the close of the Potsdam period developed topographical features of infinitely greater ruggedness than those. which now characterize that region. Kewenaw Point, the Porcupine Mountains, and the Huron Mountains, as well as the numberless unnamed knobs still standing throughout the region, have been gnawed and battered down for hundreds of feet, and their once angular outlines have been scoured to a subdued rotundity. The Appalachians, that once lifted their multiplied folds to the heights of the An

des, have been planed down to the level of third-rate mountains, and the dust and rubbish scraped from their worn. heads has been deposited in the troughs between the ridges, or strewn along the coast to form the foundations of the Atlantic States.

The city of Cincinnati stands upon the exposed core of the broadest and most westerly of these Appalachian folds. The rocky wrappings of this axis have been planed down from the summit till the Carboniferous, Devonian, Upper Silurian, and Lower Silurian strata have been successively reached, and these superincumbent layers tilt in all directions from the summit of the arch. The graves of encrinites and brachiopods, that had lain undisturbed for untold geological cycles, buried a thousand feet beneath oceanslime and careering waves, and, at a later period, beneath the roots of Carboniferous tree-ferns and the mire of steaming jungles-graves of populous nations of the olden time -were uncovered and plowed from their locations, and crumbling skeletons were strewn over the area of three states. So the spirit of modern improvement sometimes. lays its remorseless hand on the cemeteries of man, and sunlight falls again upon relics that had once been laid away with sacred care. What veneration fills the mind of the geologist as he walks over this waste of the Silurian burial-vaults. The Ohio has plowed its way through the buried city, and a city of the living has been reared upon the bones of the dead. The native ramparts which wall in the Queen City upon every side are but the broken tiers of vaults in this ancient cemetery. The area of this desecration extends to Madison and Richmond in Indiana, and to Danville and Richmond in Kentucky. Throughout this entire extent of country the once superincumbent formations have been swept away, and the material wrought into the structure of formations of later date.

I ascend to the cupola of the magnificent state-house at Nashville, and take a survey of the surrounding country. On every side spread out the broadly undulating fields of grass and corn into the illimitable distance. A finer agricultural scene was never witnessed. A more beautiful landscape, diversified with broad clearings, waving crops, tufts of magnolia and poplar, shining mansions, withdrawing vales, and purple atmosphere, it has never been my privilege to gaze upon. What is the substratum of all this beauty of form and landscape? Descending to the ground, I find myself standing again upon the opened sepulchres of Lower Silurian populations. I go down to the bank of the Cumberland, and view the sharp-cut walls which frown above the muddy current a hundred feet below. Here is a deep perpendicular gorge chiseled by the river through the marble strata of the Trenton and Cincinnati groups. I set out upon an exploration of the charming country mapped before me from the dome of the Capitol. Traveling eastward for sixty miles, I pass continuously over an undulating exposure of the same strata. Here I find an outer wall four hundred feet high, which bounds this magnificent basin of Middle Tennessee on every side (Fig. 95). I climb to the top of this wall, and ascertain that it is at this point, the western termination of a series of overlying strata of Silurian and Devonian age, which to the west have been swept away, but toward the east form an elevated plateau, through which the streams have scored deep gorges four hundred feet down to the level of the central basin.

This "highland rim," as my scientific friend, Professor Safford, styles it, is forty miles wide. We come then to the foot of the Cumberland Mountains-or, more properly, Cumberland Table-land-and ascend a thousand feet over the outcropping edges of Lower Carboniferous strata, and

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