CATSKILL GROUP. A series of sandstones and shales of Upper Devonian age exposed along the western slopes of the northern Appalachian Mountains and named from the Catskill :Moun tains of New York, where they were first studied. The series was formerly supposed to constitute a distinct geological group overlying the Chemung group, but careful investigation has shown that the rocks of the Catskill formation are shallow water deposits formed along the shorelines of the northeast bay of the interior Devonian sea during periods while the normal marine sedi ments of the Hamilton, Portage, and Chemung groups were being laid down in the open and deeper waters that filled the western parts of the same bay. The name 'Catskill formation,' then, signifies a local littoral development or facies of the normal Upper Devonian formations, and as such it has no place in the geological time-scale. These Catskill conditions and sediments began in the Devonian of the eastern border of the basin at an earlier period than they did in the central and western portions, for the shoaling of the Devonian sea progressed from east to west. Be cause of this the Lower Catskill deposits of the Catskill Mountain region are of Ilamilton age, the middle Catskill of the eastern border and the Catskill of the east-central part of New York State are of Portage age and known as the Oneonta formation, while the Upper Catskill of the eastern border and the Catskill rocks of the central part of the State are of Chemung age.
The rocks of the Catskill formation consist of shale and sandstone, with the latter often grad ing into coarse conglomerates, and their pre dominant colors are red, brown, greenish, and steel-gray. The sandstone, especially that of steel-gray color, often splits readily into thin horizontal layers and is then quarried for flag stone. The entire output of flagstone known as `Hudson River bluestone' is obtained from the Catskill formation of Ulster, Delaware, and Greene counties, N. Y. As a rule the Cats
kill rocks are poor in fossil remains. Those most commonly found are the remains of land plants such as ferns, lepidodendrons, and trunks of coniferous trees. The most noteworthy fossil from this formation is a gigantic arthropod, (q.v.), an ally of the horseshoe crab (Limulus), which attained a length of five feet. Fragments, consisting of the dermal plates. spines, and teeth, of heavily armored fishes of the genera Bothriolepis, Coccosteus, and Holopty chins, which lived in the brackish and fresh waters of the coastal swamps, are found in cer tain red beds. A fresh-water clam. Amnigenia, is found in sandy shales of the Oneonta forma tion and occasional water-worn shells of marine molhasks and brachiopods occur in the lower beds of the Catskill Mountain region.
The Catskill sediments are approximately 4500 feet thick in the Catskill Mountains: they have their greatest developthent, with a thickness of 7500 feet, in the vicinity of Mauch Chunk, Pa., and from that region southward the formation diminishes in thickness until it disappears in Virginia. The formation is overlaid by the Pocono sandstones and conglomerates of similar origin, but of later (Carboniferous) age. The elevation of the continents that occurred toward the close of Devonian time was accompanied in Great Britain by the formation of very similar sediments there known as the Old Red Sandstone, made famous by the writings of Hugh This Old Red Sandstone bears to the normal marine Devonian of Great Britain the same rela tion as does the Catskill formation to the marine Devonian of eastern North America, and the sim ilarity is still further marked in the identical physical characteristic: of the rocks and the close resemblance of their contained fossil re mains. See DEVONIAN SYSTEM; OLD RED SAND. STONE; STYLONURUS.