(2) The Corniferous or Upper Helderberg Limestone.—­A series of limestones usually charged with considerable quantities of siliceous matter in the shape of hornstone or chert (Lat. cornu, horn).  The thickness of this group rarely exceeds 300 feet; but it is replete with fossils, more especially with the remains of corals.  The Corniferous Limestone is the equivalent of the coral-bearing limestones of the Middle Devonian of Devonshire and the great “Eifel Limestone” of Germany.

(3) The Hamilton Group—­consisting of shales at the base ("Marcellus shales"); flags, shales, and impure limestones ("Hamilton beds”) in the middle; and again a series of shales ("Genesee Slates”) at the top.  The thickness of this group varies from 200 to 1200 feet, and it is richly charged with marine fossils.

(4) The Portage Group.—­A great series of shales, flags, and shaly sandstones, with few fossils.

(5) The Chemung Group.—­Another great series of sandstones and shales, but with many fossils.  The Portage and Chemung groups may be regarded as corresponding with the Upper Devonian of Devonshire.  The Chemung beds are succeeded by a great series of red sandstones and shales—­the “Catskill Group”—­which pass conformably upwards into the Carboniferous, and which may perhaps be regarded as the equivalent of the great sandstones of the Upper Old Red in Scotland.

Throughout the entire series of Devonian deposits in North America no unconformability or physical break of any kind has hitherto been detected; nor is there any marked interruption to the current of life, though each subdivision of the series has its own fossils.  No completely natural line can thus be indicated, dividing the Devonian in this region from the Silurian on the one hand, and the Carboniferous on the other hand.  At the same time, there is the most ample evidence, both stratigraphical and palaeontological, as to the complete independence of the American Devonian series as a distinct life-system between the older Silurian and the later Carboniferous.  The subjoined section (fig. 76) shows diagrammatically the general succession of the Devonian rocks of North America.

[Illustration:  Fig. 76.  GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.]

[Illustration:  Fig. 77.—­Restoration of Psilophyton princeps.  Devonian, Canada. (After Dawson.)]

As regards the life of the Devonian period, we are now acquainted with a large and abundant terrestrial flora—­this being the first time that we have met with a land vegetation capable of reconstruction in any fulness.  By the researches of Goeppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here.  As might have been anticipated, the greater number of the vegetable remains of this period have been obtained from such shallow-water