may be considered the bottom of the Upper, or top of the Lower, Silurian series. Mr. H. D. Rogers informs me, that he and his brother have traced the scales of fish through strata of this series from the south-western part of Virginia to the north branch of the Susquehanna, in Pennsylvania.

Professor E. Forbes, after acquiring much experience, by dredging in the Mediterranean, of the fauna which characterizes the sea at different depths, has inferred that the Silurian seas in those areas hitherto examined were at first very deep and tranquil, although in parts they afterwards grew shallower. The following are the principal grounds of this conclusion:-first, the small size of the greater number of conchifera; secondly, the paucity of pectinibranchiata (or spiral univalves); thirdly, the great number of floaters, such as Bellerophon, Orthoceras, &c.; fourthly, the abundance of brachiopoda; fifthly, the absence or great rarity of fossil fish; sixthly, the deep-water forms of most of the sea-weeds; and seventhly, the absence of land plants.

50

CINCINNATI.

CHAP. XVII.

CHAPTER XVII.

Alluvial Terraces at Cincinnati, and their Origin.-Bones of the Elephant and Mastodon.-Excursion to the Swamps of Big Bone Lick, in Kentucky.-Noble Forest.-Salt Springs.Buffalo Trails.-Numerous Bones of extinct Animals.-Associated Freshwater and Land Shells.—Relative Age of Northern Drift, and Deposits with Bones of Mastodon on the Ohio

THE Ohio river at Cincinnati, and immediately above and below it, is bounded on its right bank by two terraces, on which the city is built; the streets in the upper and lower part of it standing on different levels. These terraces are composed of sand, gravel, and loam, such as the river, if blocked up by some barrier, might now be supposed to sweep down in its current, and deposit in a lake. The upper terrace, b (fig. 9), is bounded by steep hills of ancient fossiliferous rocks. A, the blue, Lower Silurian limestone, mentioned in the last chapter, in horizontal stratification. The higher terrace, b, is about 60 feet above the lower,, and this again about 60 feet above low water in the Ohio, d. The geologists here are convinced that the inferior terrace, c, is of newer origin than b, as shown in the section (fig. 9), and proved by excavations, not exposed at the time of my visit.

In sinking a well through c, at the distance of 300 yards from the Ohio, and at the depth of 50 feet from the surface, they found, at e, pieces of wood and many walnuts in a bed of silt.

Near the edge of the higher terrace, in digging a gravel-pit, which I saw open at the end of Sixth

street, they discovered lately the teeth of the Elephas primigenius, the same extinct species which is met with in very analogous situations on the banks of the Thames, and the same which was found preserved entire with its flesh in the ice of Siberia. Above the stratum from which the tooth was obtained, I observed about six feet of gravel covered by ten feet of fine yellow loam, and below it were alternations of gravel, loam, and sand, for 20 feet. But I searched in vain for any accompanying fossil shells. These, however, have been found in a similar situation at Mill Creek, near Cincinnati; a place where several teeth of mastodons have been met with. They belong to the genera Melania, Lymnæa, Amnicola, Succinea, Physa, Planorbis, Paludina, Cyclas, Helix, and Pupa, all of recent species, and nearly all known to inhabit the immediate neighbourhood. I was also informed that, near Wheeling, a bed of freshwater shells, one foot thick, of the genus Unio, is exposed at the height of. 120 feet above the mean level of the Ohio. The remains of the common American mastodon (M. giganteus) have also been found at several points in the strata in the upper terrace, both above and below Cincinnati.

52

ALLUVIAL TERRACES

CHAP. XVII.

Upon the whole it appears, that the strata of loam, clay, and gravel, forming the elevated terraces on both sides of the Ohio and its tributaries, and which we know to have remained unaltered from the era of the Indian mounds and earthworks, originated subsequently to the period of the existing mollusca, but when several quadrupeds now extinct inhabited this continent. The lower parts both of the larger and smaller valleys appear to have been filled up with a fluviatile deposit, through which the streams have subsequently cut broad and deep channels. These phenomena very closely resemble those presented by the loess, or ancient river-silt of the Rhine and its tributaries, and the theory which I formerly suggested to account for the position of the Rhenish loess (also charged with recent land and freshwater shells, and occasionally with the remains of the extinct elephant) may be applicable to the American deposits.

I imagined first a gradual movement of depression, like that now in progress on the west coast of Greenland, to lessen the fall of the waters, or the height of the land relatively to the ocean. In consequence of the land being thus lowered, the bottoms of the main and lateral valleys become filled up with fluviatile sediment, containing terrestrial and freshwater shells, in the same manner as deltas are formed where rivers meet the sea, the salt-water being excluded, in spite of continued subsidence, by the accumulation of alluvial matter, brought down incessantly from the land above. Afterwards, I suppose an upward movement gradually to restore the country to its former level, and, during this upheaval, the rivers remove a large part of the accumulated mud, sand, and gravel. I

have already shown that on the coast of Georgia and South Carolina (see Vol. I., p. 164), in the United States, we have positive proofs of modern oscillations of level, similar to those here assumed.

Two days after I reached Cincinnati, I set out, in company with two naturalists of that city, Mr. Buchanan and Mr. J. G. Anthony, who kindly offered to be my guides, in an excursion to a place of great geological celebrity in the neighbouring State of Kentucky, called Big Bone Lick, where the bones of mastodons and many other extinct quadrupeds had been dug up in extraordinary abundance. Having crossed the river from Cincinnati, we passed through a forest far more magnificent for the size and variety of its trees than any we had before seen. The tulip-tree (Liliodendron tulipiferum) the buckeye, a kind of horse-chestnut, the shagbark hickory, the beech, the oak, the elm, the chestnut, the locust-tree, the sugar-maple, and the willow, were in perfection but no coniferous trees,-none of the long-leaved pines of the Southern Atlantic border, nor the cypress, cedar, and hemlock of other States. These forests, where there is no undergrowth, are called "wood pastures." Originally the cane covered the ground, but when it was eaten down by the cattle, no new crop could get up, and it was replaced by grass alone.

Big Bone Lick is distant from Cincinnati about twenty-three miles in a S.W. direction. The intervening country is composed of the blue argillaceous limestone and marl before mentioned, the beds of which are nearly horizontal, and form flat table-lands intersected by valleys of moderate depth. In one of