Page:The Journal of geology (IA journalofgeology21894univers).pdf/454

this subsidence the Columbia formation was deposited. Some of its characters are similar to those of the Lafayette, and indeed the latter deposit may often be mistaken for the earlier, where unconformity is not apparent. The Columbia formation covered the lower half of the coastal plain, and partly filled the great valleys which thus became estuaries. These deposits form the "second bottoms" of many of the coastal rivers, particularly on the Gulf slope. In short, the Columbia formation of the South is largely the Lafayette made over, though in the North its materials grade into those of the glacial period.

Following the Columbia submergence the continental margin again rose, even to an altitude above that of modern times, to such an extent as to permit of the clearing out of the valleys to a considerable extent; including those now submerged along the oceanic plateau. Then followed a subsidence to modern conditions. This post-Columbia elevation did not last nearly so long as the post-Lafayette, for 90 per cent. of the accumulations still remain.

The altitudes at which the Lafayette deposits are now found vary. In Maryland they occur at 500 feet; southward they decline so that at Hatteras they occur at 100-200 feet. Along the axis of greatest oscillation in South Carolina the formation rises to 800 feet, but again descends southward so that north of Mobile Bay they rise only 500 feet above tide. Again in Illinois and Arkansas, the loams rise to only 350 and 250, whilst they culminate at 1,000 feet along the Rio Grande. But as river terraces of the streams emptying into the Lafayette sea, the reviewer has met with the extension of the formation in the southern Appalachian at 1,500 to 2,000 feet, thus supporting the author's conclusions as to the greater magnitude of terrestrial undulation in the mountain regions than along the coast.

At Cape Hatteras, the Columbia deposits now rise only 25 feet above tide, but they increase to 300 feet in altitude to the north and again southward, so that in South Carolina they rise to 650 feet. Again they decline to 25 feet above the Gulf in Mobile Bay. Farther southwestward their present elevation is from 100 to 200 feet.

The meager flora of the Lafayette has both Cretaceous and Pleistocene features, and the more meager fauna represents the entire Neocene. The Columbia is regarded as the earliest Pleistocene, and the Lafayette as the later Pliocene, though the author groups it with the Miocene and small areas of marine Pliocene, the whole making the American Neocene. Its biological relations are not known; it is by