A remnant of one of the old channels is marked by a coralline limestone in northern Vermont.

The Onondaga limestone is largely a coral formation, and in some places the reefs are still beautifully preserved, as in the famous example at the Falls of the Ohio, above Louisville, Ky.

Limestone formation on such a scale implies the very general absence of terrigenous sediments from the epicontinental seas which may be interpreted as due in part to the base-levelling of the surrounding lands.

A change of conditions in the northeastern bay of the Interior Sea, probably the elevation of the land and resulting rejuvenation of the streams, checked the accumulation of limestone and brought in great quantities of mud and silt (Marcellus and Hamilton), though in the Mississippi valley limestone-making still went on, and even in New York thin limestones occur here and there in the great mass of the Hamilton shales. At Gaspe continental sedimentation continued, and there the Erian series is represented by a very thick mass of sandstones, bearing fossils of land plants, and showing occasional brief incursions of the sea. . In New Brunswick and Nova Scotia are sandstones and shales. Western Maryland and the adjoining parts of the Virginias were again submerged, but this time apparently by an expansion of the Interior Sea, that seems to have covered part of the old Cumberland Basin, which had emerged at the end of the Oriskanian. In the West the seas remained much as they had been during the Onondaga; indeed, the difference in interpretations of the latter stage, referred to on the preceding page, is chiefly a question of distinguishing Hamilton from Onondaga. A depression submerged the coast of British Columbia up to Alaska.

The Upper Devonian has much the same distribution as the Middle, but the Interior Sea appears to have lost its connection with the Gulf of Mexico and to have become joined with the Dakota Sea, while here and there in the Mississippi valley, the Upper Devonian overlaps older rocks where the Lower and Middle are absent. The Tully stage is a locally developed limestone in New York, which is of interest as corresponding to the Cuboides zone in the Rhenish section; both contain the very widespread brachiopod species, Hypothyris cuboides, a very useful guide or index fossil, as marking the base of the Upper Devonian. The Genesee is a mass of black shales, which increases in thickness from Lake Erie to central Pennsylvania, where it is 300 feet thick, and is succeeded by the Portage, which is largely arenaceous. The Portage exceeds 1000 feet in thickness. In western New York both the Genesee and the Portage have embedded in them a limestone which carries a highly interesting assemblage of animals called the "Naples Fauna." This has very little in common with that of the Hamilton, but is well marked in many parts of the world. (Figs. 255-6, pp. 522-3.) "The migration path of this pelagic fauna has been traced toward the Northwest through Manitoba into Siberia, thence through Russia into Westphalia. . . . In New York, where its fauna became extensive, it was alien and short-lived." (Clarke).

The Chemung is a great mass of sandstones and conglomerates, which reaches its maximum thickness (8000 feet) in Pennsylvania, thinning greatly to the westward. Indeed, the stages of the Upper Devonian given in the New York scale, can seldom be recognized except in that State and in Pennsylvania. In Ohio the whole Upper Devonian is represented by the Ohio Shale, which varies from 300 to 2600 feet in thickness. The Catskill, which was originally regarded as a distinct series, is a very thick mass of sandstones, representing a facies of the Upper Devonian extending through the Senecan and Chautauquan epochs and, in Pennsylvania, into the earliest Carboniferous. These beds are believed to have been accumulated in a long and narrow estuary, running from eastern New York into Pennsylvania, where the beds reach a thickness of 7500 feet, and containing fresh water in part of its course. Areas of similar rocks, with fresh or brackish water fossils, occur in the Portage and Genesee and represent deposition in coastal lagoons.

The Western Devonian indicates such a different succession of physical events, that its subdivisions can seldom be correlated with those of the East. Devonian strata are not known to underlie the Great Plains, and the Front Range of Colorado appears to have been a land area, for there Carboniferous strata overlap and rest upon Cambrian and Ordovician. On the other hand, in the plateau region, from Arizona to Montana and along the Canadian Rockies are many Devonian .outcrops, indicating that much or all of this region was submerged at one epoch or another of the period. Oscillations of level are also shown, as in the Grand Canon region, where thin and worn patches of Devonian, evidently remnants of a much thicker and more widespread mass, lie unconformably upon Cambrian and below Carboniferous beds. In the Wasatch Mountains of eastern Utah the Devonian is represented by 2400 feet of quartzites and limestones, and in the Nevada trough, where deposition seems to have been unbroken, 2000 feet of shale and 6000 feet of limestone are assigned to this system. Though the faunas differ notably from those of the East and have more affinity with those of Europe and Asia, correspondences with the Helderberg, Onondaga, and Hamilton have been observed.

Affinities with the Old World are also shown by the Devonian of northern and southern California, British Columbia, and Alaska.

Like the rocks of the other Palaeozoic systems in North America, those of the Devonian are quite free from igneous intrusions, except in several widely separated localities. Contemporaneous lava flows are interbedded with the Lower Devonian shales of northern New Brunswick, pointing to volcanic eruptions in that region, and the same is true regarding the Devonian of Nova Scotia, and that of northern California has beds of tuff and lava sheets. In central New York and various places in the West the Devonian strata are cut by intrusions, but these may be post-Devonian in date.

Comparing the rocks of the Ordovician, Silurian, and Devonian, as these are developed in the Appalachian and adjoining regions, a certain rhythmic or periodic recurrence of events may be discovered among them. Each system is characterized by a great and very widespread limestone formation, the Trenton, Lockport-Guelph, and Onondaga, respectively, and in each the limestone is succeeded by shales or other clastic rocks, the Utica, Salina, and Portage, due to an increase of terrigenous material, and each was closed by a more or less widespread emergence of the sea-bottom. Each began with a subsidence which gradually extended to a maximum at the time when the great limestone was formed. The parallelisms are not exact, but they are certainly suggestive.