The Salina group, one of the upper members of the Cayugan, yields the salt obtained from wells in New York, Ohio and Ontario ; and the Clinton formation, the basal member of the Niagaran, yields hematite, or red iron ore, from New York to Alabama, now worked extensively only in the Birmingham region, and consider able oil and gas in central Ohio and eastern Kentucky.
The Devonian system is widely distributed in North America, but the lower groups are confined to the margins of the continent, whereas the middle groups also occupy a considerable portion of the interior of the continent, where the disconformity in faunal succession makes it easy to separate the Devonian from the underlying Silurian. The Catskills constitute one of the most impressive sections of the Devonian in the United States, and the Eureka district of Nevada another. Excellent type sections also outcrop along the shore of Lake Erie for about 25 m. west of Buffalo, and about Cumberland, Md. Cleveland, Ohio, is situated on Upper Devonian; Sandusky and Columbus, in the same State; Milwaukee, Wisconsin, Davenport, Iowa and Alpena, Michigan, are situated upon Middle Devonian.
When the Devonian period opened, practically all of North America had emerged from the ocean, only narrow seas occupying the Appalachian, Acadian and Cordilleran troughs, and an embay ment extending northward from the Gulf to the base of the Cincin nati uplift and the Ozark dome. The continent began submerging early, however, and by the Middle Devonian the Gulf embay ment with its warm waters had spread northward to Hudson bay, and the C9rdilleran trough was occupied by a widened sea that extended northward to the Arctic. During the Upper Devonian, the seas gradually withdrew again until the interior and the Cordilleran area were no longer inundated. Only small crustal movements, except the notable Shickshockian disturbance which affected eastern Canada and north-eastern United States, and little volcanic activity distinguish the Devonian of North America.
The life of the Devonian in America may be arranged in three provinces : the Atlantic, especially in the Lower Devonian, with its life forms related to those of the Rhine district of Europe; the Central Interior, or Gulf, its life related to that of South America; and the Cordilleran, its life derived from the Pacific and the Arctic. In general, the life resembled that of the Silurian, ex
cept that fishes had become exceedingly abundant, and amphibia had begun to appear, indicative of the evolutionary trend toward vertebrate dominance, and the further rapid development of air breathing animals to occupy the land and the air.
There is no more significant ox picturesque period in the history of the earth than the Devonian. This is the time when the formor nakedness of the lands becomes clothed with a deeper verdure and the first forests appear, providing the needed homes and food for the invasion of the continents by the ever-hungry descendants of the denizens of the sea. The conquest is first attained by the invertebrates —the scorpions, shell-fish, worms and insects. Of greater significance, however, is the adaptation of other marine animals to the land, which eventually leads to the origin of the fishes, the advance guard of the vertebrate hosts, through the development of lungs and the increase in mentality.
The invasion of the land is fairly under way in the Devonian, chiefly in the rivers and lakes, but due to the prevailingly arid climates a fierce struggle is instituted among the inhabitants of the then tempo rary waters, resulting in the dominancy of the better equipped air breathing fishes, an issue prophetic of vertebrate ascendancy, hereafter never to be questioned in its onward sweep to its culmination in man. (Pirsson and Schuchert, Text-book of Geology, p. 7i4.) Petroleum and natural gas in large quantity have been derived from the Devonian formations of Pennsylvania, West Virginia, Ohio and New York.
The Carboniferous periods, including the Mississippian (Lower Carboniferous), the Pennsylvanian (Upper Carboniferous) and the Permian, are best considered together, because while there is general unconformity between the Mississippian and the Penn sylvanian, the only areas of continuous sedimentation persisting in Utah and Arizona, the transition from Pennsylvanian to Per mian is so gradual as to make definite separation impracticable, and the evolution of life forms is continuous and interrelated through the whole period.