NORTH AMERICA The seas on the east and west of the position of the present continent of North America were regions of sedimentation and subsidence, and from them the marine transgression spread over the intervening area. The succession in New York State where the beds have been but little disturbed, is given in the table. The Helderbergian consists of deep sea limestones similar to those of the Lower Devonian of Bohemia. It rests conformably on the Silurian, and is succeeded in the east by the Oriskany sandstone which occurs discontinuously as far west as the Mississippi. It resembles the Lower Devonian of the Meuse and Rhine. As the sea deepened and extended, the Onondaga limestone was laid down, which the presence of Spirifer acuminatus (cultrijugatus) would place low in the Eifelian. The limestone is represented by shales in the south, and is later itself replaced by the Marcellus shales which are correlated by their goniatites with the Wissenbach shales of Europe. They are followed by the more calcareous Hamilton shales, with increasing depth of the sea, which, at the end of the Middle Devonian, occupied 38% of the area of the present continent. Although the Hamilton was contemporaneous with the Givetian of Europe, the fauna is quite distinct from that of the Stringocephalus beds, which is, however, found in Manitoba. The affinities of the Hamilton are with southern areas.
The Upper Devonian commences with the Tully limestone, the equivalent of the Iberg limestone of Europe, followed by the Genesee bituminous and pyritous shale and the Portage beds with three facies; in the west the deep water Naples shales with Gephyroceras intumescens; in the east the Ithaca shales and sand stones with Tropidoleptus carinatus, also found in the Hamilton; and still further east the Oneonta. The Famennian is represented by the continental Catskill beds and the marine Chemung, with Spirifer verneuili. In Montana shaly marls with Clymenia and Cheiloceras indicate deeper water.
The most complete succession is found in Bolivia. Resting unconformably on the Ordovician are (I) the Icla sandstone with Silurian fossils near its base, (2) the sandy Icla shales, (3) the Huampampa sandstone, and (4) the Sicasica beds. The fauna of the Icla shales and the Huampampa sandstone is widespread through the Southern Hemisphere—in Parana (Brazil), the Argen tine, the Falklands, and the Bokkeveld beds, which overlie the Table Mountain sandstone in South Africa. Spirifer antarcticus, Chonetes falklandicus, and Leptocoelia flabellites are found throughout. The fauna of the Sicasica beds is essentially Middle Devonian with some lower forms. It has affinities with the Hamil ton. The Devonian fossils of the Sahara are related to North and South American forms. The Devonian of the Gold Coast appears to be a littoral facies of the Hamilton, and is linked with the Sicasica by Homalonotus dehayi.
The older southern fauna extending from the Sicasica shales to the Bokkeveld beds may be placed at the base of the Middle Devonian or high in the Lower. Its special characters, the scarcity of corals and bryozoa, the number of discinoid brachiopods, the absence of limestones and the dark grey, blue and green colour ing have been attributed to polar conditions, a view supported by the occurrence of glaciation in the Table Mountain sandstone. Some indeterminate lamellibranchs at the base of the latter may be Silurian like the Icla sandstone, and not Devonian. The Bok keveld passes up into the Witteberg, a series of sands and quartz ites containing indistinct plant remains, which are believed to represent a passage into the Carboniferous.
Except for the Caledonian movement at its commencement the Devonian was a time of comparative quiet, but of slow move ments, affecting the distribution of land and water and causing unconformities and overlaps.
The granite or granodiorite intrusions of the late Silurian in the north of Great Britain continued into the early Devonian. They were associated with volcanic activity, mainly of an andesitic or basaltic character, and minor intrusions of similar composition. In Devon and Cornwall there were, in later Devonian times, sub marine outpourings of spilitic pillow lavas accompanied by tuffs.
They resemble the contemporaneous schalsteins of Germany There are iron ores of some importance mostly occurring as replacements of limestone. The metalliferous lodes of Cornwall and Devon are largely in rocks of Devonian age, but are the result of Armorican igneous activity. The oil of Ontario has its source in Middle Devonian shales, while that of New York and Pennsyl vania may be derived from the black shales of the Upper Devonian.