CHAP. XXXIV.] OSCILLATION OF LEVEL. 197 



of scouring 1 out and deepening their channels, would be greater 

 and greater, till at length, after a lapse of many thousand years, 

 each of them would have eroded a deep channel or valley through 

 the fluviatile formation previously accumulated. The surface of 

 what was once the river-plain at the period of greatest depression, 

 would remain fringing the valley sides as a terrace, apparently 

 flat, but in reality sloping down with the general inclination of 

 the valley. Every where this terrace would present clifis of 

 gravel and sand facing the river. 



After these changes, the fundamental strata (Nos. 3,4, 5, fig. 

 1 1, p. 196) might be restored nearly to their ancient positions ; 

 the fresh- water beds (No. 2) having been raised, and having suf 

 fered great denudation. 



It is not improbable that the same series of movements gave 

 rise to the accumulation and present position of marine strata of 

 comparatively modern date, forming the lower terrace near Da- 

 rien in Georgia* 1 which is indicated at 2*, in the annexed section 

 (fig. 11). The reader will remember that the remains of the 

 megatherium, mastodon, elephant, Harlanus, equus, and other ex 

 tinct species of land quadrupeds, are there associated with marine 

 shells, of species agreeing with those now inhabiting the Atlantic. 



On the other hand, there are proofs in Texas of the prevalence 

 of the same succession of subterranean movements far to the south 

 west, along the country bordering the Gulf of Mexico ; for on the 

 Brazos River there are beds of loam, or loess, examined by Dr. 

 Dickeson, and, when at New Orleans, I saw the bones of extinct 

 quadrupeds brought from that deposit. Among them was the 

 jaAv-bone of a tapir, apparently identical with the South Ameri 

 can species ; remains of the mastodon, elephant, ox, and other 

 mammalia, much resembling, on the whole, those found at Nat 

 chez and on the Ohio. 



As to the seaward extremity of the ancient delta, the effect of 

 the gradual depression of land above assumed would be to cause 

 its mud and sand to increase in thickness, instead of augmenting 

 in area. When at length the movement was reversed, and the 

 fresh- water deposits began to rise, the action of the sea would un- 

 * See ante, vol. i. p. 257. 



