Devonian, the name of one of the geologic ages, the age of fishes, and the second of the three ancient or palaeozoic divisions of time. It followed the Silurian, or age of mollusks, which till recently was thought to contain the earliest vestiges of organic life, and preceded the carboniferous. These three ages, constituting the palaeozoic era, were followed by the age of reptiles, which constitutes the mesozoic era. The Devonian age, or Devonian formation, as the rocks are called, was named from Devonshire in England by Sir R. Murchison and Prof. Sedgwick, who about the year 1837 distinguished its strata from those of the Silurian below and the carboniferous above. The transition of the Silurian to the Devonian formation is gradual and easy, and sometimes rather difficult to determine; so much so that differences of opinion exist in regard to some of the strata in certain localities; but a broad distinction in the two ages is marked by the forms of the development of life. The periods and epochs into which the Devonian age is divided, according to the system of the New York state geologists, are as follows;

The first and second periods are often called the lower Devonian, and those above,the upper Devonian. The corniferous was the great limestone period of America. Above it shales and sandstones predominate, the limestone beds being subordinate. The Oriskany formation, named from Oriskany, Oneida co., N. Y., is about 30 ft. thick at that place, composed mainly of rough sandstones. Along the Alle-ghanies it extends through Pennsylvania, Maryland, and Virginia, and in these states often reaches a thickness of several hundred feet. No land plants have been found among its fossils, and the mass of evidence points to the non-existence of land vegetation during the Oriskany period. The most common species of animals are the spirifer arenosus and Rens-selaeria ovoides, their large fossil shells being often crowded together, and composing a good share of the rock. The cauda galli epoch of the corniferous period is named from the feathery forms of a fossil, supposed to be the impressions of a seaweed. The rock is principally argillaceous sandstone, and in the Helderberg mountains, near Albany, N. Y., is from 50 to 60 ft. thick. The rocks of the Schoharie epoch are principally fine-grained, calcareous sandstones, full of fossils. In New York the beds are all in the eastern part of the state.

The rocks of the upper Helderberg epoch are limestones, and are widely distributed over the interior continental basin from New York to beyond the Mississippi. In New York they are divided into Onondaga and corniferous limestone. This latter, from which the period takes its name, is called corniferous because it contains masses of hornstone or imperfect flint. The plants of this period are seaweeds and protophytes, the cauda galii being among the former. The upper Helderberg epoch is the coral reef period of the palaeozoic ages, abounding in corals, some of which are found standing in the position in which they grew, but they are generally more or less comminuted. This formation attains in some places a thickness of 350 ft. The corniferous period is especially remarkable for containing the earliest discovered remains of fishes, the first development of vertebrate animals. The oldest development of them has. been found in the United States, in the Schoharie grit. The Devonian formation contains two of the great divisions of fishes, the sala-chians or sharks, and the ganoids, of which the gar-pike and sturgeon are representatives. The Marcellus shale of the Hamilton period is a soft argillaceous rock, containing sufficient traces of coal to afford a flame when placed in a fire.

The Hamilton beds contain shale and flagging stone, and are overlaid by the black Genesee shale. The Hamilton beds are remarkable for containing numerous ripple marks, and for having the strata intersected by regular joints; fine examples of which are found near Cayuga lake, N. Y. These beds contain fine fossils of gasteropods, cephalopods, and trilobites. The Hamilton formation extends across New York from Lake Erie east, having its greatest thickness, about 1,200 ft., east of the centre. It extends into Michigan, Illinois, and Iowa, in thinner strata. The rocks of the Portage epoch of the Chemung period have a thickness of 1,000 ft. on the Genesee river, and of 1,400 ft. near Lake Erie, but are not found in eastern New York. The Chemung group extends over the southern tier of counties in New York, attaining in places a thickness of 1,500 ft. It abounds in organic remains, both vegetable and animal, containing, besides the cauda galli seaweed, numerous land plants, and many species of crinoids, brachio-pods, conchifers, bellerophons, and goniatites. The last period of the Devonian formation, the Catskill, is composed mainly of shales and sandstones, the latter predominating, passing into conglomerates particularly in the upper formations.

There are ripple marks and other signs of wave action. The vestiges of animal life are fewer than in the earlier periods, and widely differ from them in character. No corals, crinoids, brachiopods, or trilobites have been found. There are a few conchifers and fragments of fishes, some of which were of large size, the fins being a foot in length. The beds, however, have not been fully explored. The land plants are of much the same character with those of the Chemung period. A frond of one of the characteristic ferns, found at Montrose, Pa., was more than a foot in breadth. The Catskill formation is thin in the western part of New York, but along the Hudson river, in the Catskill mountains, it attains a thickness of 2,000 or 3,000 ft. It passes beneath the coal formation in Pennsylvania and Virginia, attaining in the Appalachian region a thickness of 5,000 or 6,000 ft. - The Devonian rocks appear at the surface in most parts of all the continents; in Great Britain they appear in Wales, Herefordshire, Devonshire, and Cornwall, and are also found in Ireland and the Isle of Man; but they are most developed in the United States.