With the end of the Silurian period the _Age of Invertebrates_ comes to a close, giving place to the Devonian, the _Age of Fishes_. [Illustration: Fig. 295. Block of Limestone showing Interior Casts of _Pentamerus oblongus_, a Common Silurian Brachiopod] CHAPTER XVIII THE DEVONIAN In America the Silurian is not separated from the Devonian by any mountain-making deformation or continental uplift. The one period passed quietly into the other. Their conformable systems are so closely related, and the change in their faunas is so gradual, that geologists are not agreed as to the precise horizon which divides them. =Subdivisions and physical geography.= The Devonian is represented in New York and southward by the following five series. We add the rocks of which they are chiefly composed. 5 Chemung ..... sandstones and sandy shales 4 Hamilton ..... shales and sandstones 3 Corniferous ..... limestones 2 Oriskany ..... sandstones 1 Helderberg ..... limestones The Helderberg is a transition epoch referred by some geologists to the Silurian. The thin sandstones of the Oriskany mark an epoch when waves worked over the deposits of former coastal plains. The limestones of the Corniferous testify to a warm and clear wide sea which extended from the Hudson to beyond the Mississippi. Corals throve luxuriantly, and their remains, with those of mollusks and other lime-secreting animals, built up great beds of limestone. The bordering continents, as during the later Silurian, must now have been monotonous lowlands which sent down little of even the finest waste to the sea. In the Hamilton the clear seas of the previous epoch became clouded with mud. The immense deposits of coarse sandstones and sandy shales of the Chemung, which are found off what was at the time the west coast of Appalachia, prove an uplift of that ancient continent. The Chemung series extends from the Catskill Mountains to northeastern Ohio and south to northeastern Tennessee, covering an area of not less than a hundred thousand square miles. In eastern New York it attains three thousand feet in thickness; in Pennsylvania it reaches the enormous thickness of two miles; but it rapidly thins to the west. Everywhere the Chemung is made of thin beds of rapidly alternating coarse and fine sands and clays, with an occasional pebble layer, and hence is a shallow-water deposit. The fine material has not been thoroughly winnowed from the coarse by the long action of strong waves and tides. The sands and clays have undergone little more sorting than is done by rivers. We must regard the Chemung sandstones as deposits made at the mouths of swift, turbid rivers in such great amount that they could be little sorted and distributed by waves.