The name Devonian, taken from the English county Devonshire, was proposed by Sedgwick and Murchison in 1839; it has found universal acceptance and has passed into the geological literature of all languages.
As in the Ordovician and Silurian, and for the same reason, the divisions of the New York Devonian are taken as the standard of reference for North America, but the general standard is no longer to be found in Great Britain, in spite of the first description of these rocks in Devonshire. This is because these rocks in England are so much disturbed, metamorphosed, and faulted, that the order of succession among them could not be determined until the Devonian of the Rhine and Belgium, the largest and best-developed Devonian area of western Europe, had first been made out. Hence, the Rhenish section is widely employed as a standard for the different continents.
Port Ewen Stage
New Scotland Stage
In certain areas, notably in that of the Cumberland Basin, the transition from Silurian to Devonian is so gradual that the boundary between them remained long in doubt and has been shifted more than once. The Helderbergian series has, until quite recently, been very generally referred to the Silurian, and at one time even the Oriskanian was included in the same system.
At the beginning of the Devonian (Helderbergian epoch) most of the continent west of the Cumberland Basin was land, but the Basin itself continued to deepen and enlarge, and subsidence in the South brought the sea in over western Tennessee into Missouri and southern Illinois, reaching into Oklahoma. The limestones of the Helderbergian series, which were laid down in the Cumberland Basin, extend, with some interruptions, from southwestern Virginia along the line of the Appalachians to Albany, N. Y. The Gaspe Peninsula, which forms part of the west coast of the Gulf of St. Lawrence, was submerged at this time, for here we find 1500 feet of limestone representing the Helderbergian and Oriskanian series. " The St. Lawrence tidal waters of this period must have extended westward to the border of Vermont and Montreal and southward along the Connecticut valley." (Dana.) Northern and southern New Brunswick, northern Nova Scotia, northern Maine and part of its coast were under the Helderbergian sea, and the New England troughs, the Connecticut valley, and the Gaspe-Worcester trough were submerged.
In the West the Helderbergian has been identified only in the Nevada trough, where marine sedimentation continued uninterruptedly from the Silurian, while in the far North on the shore of Kennedy Channel, 8o° N. lat., Helderbergian fossils have been found.
Fig. 265. - Map of North America in the Devonian period. Black areas = known exposures; white areas = land, or unknown; horizontal lines = sea.
The Oriskanian epoch witnessed some geographical changes. The rocks of this series, which are prevailingly sandstones laid down in the Cumberland Basin, are thickest in western Maryland, and thin away both north and south from that region; to the south the beds are chiefly lower Oriskany and to the north mostly upper Oriskany, indicating oscillations of the Basin floor. In central New York the Oriskanian waters extended themselves westward across the state, breaking through the barrier in the southeast. In the Mississippi valley, the Oriskany has about the same limits as the Helderbergian, but it has not been found in Oklahoma, while, on the other hand, the sea transgressed over northern Georgia and Alabama, where the Oriskany sandstone, only 20 feet thick, successively overlaps the older formations from the Middle Cambrian to the late Ordovician. In addition to the Oriskanian limestones of the Gaspe Peninsula, rocks of this stage are found in Nova Scotia and New Brunswick and northern Maine, in which State they are enormously thick, 5000 feet.
The Lower Devonian of the Maritime Provinces, and especially of Maine, shows clear indications of an invasion of the Coblenzian fauna of Europe, from which may be inferred the existence of a land bridge across the North Atlantic, affording the necessary conditions for the migration of the shoal-water animals. "The evidence then is fairly conclusive that during the period represented by the Coblenzian-Oriskany, the arenaceous epicontinental sediments was the ground traversed by the Coblenz fauna westward along the North Atlantic continent." (Clarke).
Very extensive changes characterized the Middle Devonian; the Cumberland Basin was elevated into land at the end of the Oriskany, and in eastern New York we have only the coarse sands 2 o and grits of the Esopus (which may be a phase of the Oriskany) and the Schoharie, but the Interior Sea was once more established, with a restricted area, and not improbably connecting with the Gulf of St. Lawrence by way of the Connecticut valley trough. In it was accumulated the great Onondaga limestone, which extends from the Hudson River across New York into Michigan, and around what may have been the islands of the Cincinnati anticline into Indiana, Illinois, and Kentucky. Interpretations differ concerning the form and size of this Mississippi valley sea; according to one view, it extended southward to the Gulf of Mexico, while another makes it completely enclosed on the south and postulates a northern extension across Hudson's Bay to the Arctic Sea. Another great invasion, called the Dakota Sea, which may have had no direct communication with the former, opened from the Arctic Ocean, where now is the mouth of the Mackenzie River, and crossed British America, the Dakotas, Nebraska, and Kansas, sending a gulf into Iowa, Minnesota, and northern Illinois and southern Wisconsin, and reaching or perhaps passing through Texas. " This channel was bounded on the west by the extensive Archaean islands or edges of land constituting the eastern axis of the present Rocky Mountains." (Williams.) However, another interpretation separates the Dakota Sea from the Mississip-pian by the width of the Great Plains, with a possible cross-connection in southern Canada, and includes all the eastern Onondaga in the Interior Sea. A third sea, the Cordilleran, covered much of the Great Basin, probably extending to the Pacific and connecting on the north with the Dakota Sea. In the northeastern part of the continent the Gaspe area was converted into a coastal lagoon, into which great masses of sand were swept by rapid streams; these sands contain numerous land plants.