(1) _Cauda-Galli Grit_ and _Schoharie Grit_.--Considering the "Oriskany Sandstone" as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire. The _Cauda-Galli Grit_ is so called from the abundance of a peculiar spiral fossil (_Spirophyton cauda-Galli_), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed. (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 palæontological, 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.)]