The salient features of the physical history of the next, or Devonian period, are much like those of the preceding Silurian. At the beginning of the Devonian almost all of the continent was dry land, but soon a long, narrow arm of the sea extended across the eastern side of the continent from the Gulf of St. Lawrence southward through western New England, southeastern New York and throughout the Appalachian district, thus reminding us of the long, narrow sound which occupied almost exactly the same territory during the early part of the Cambrian period. In the west the only water was a small embayment reaching across southern California into Nevada. By middle Devonian time these water areas had considerably expanded. During relatively late Devonian time the sea was so expanded as to cover much of the Mississippi Valley area, the Appalachian Mountains and St. Lawrence Valley areas, and most of the site of the Rocky Mountains, except for an island of considerable size reaching from New Mexico through Wyoming. The main lands were most of northeastern North America, a large land area extending from Florida to Nova Scotia, and a large area on the western side of the continent from California to Alaska. A remarkable formation of late Devonian Age should be briefly described. In southeastern New York and the northern Appalachian region there was a tremendous accumulation of sediments which have consolidated into sandstone, together with some shale and conglomerate. This so-called "Catskill" formation is from 1,500 to 8,000 feet thick and is well shown as the main body of rock in the Catskill Mountains. It is largely a shallow-water deposit of essentially nonmarine origin, as proved by coarseness of material, ripple marks, and nonmarine fossils. All evidence points to the origin of this remarkable formation as a great delta deposit built out into the shallow interior sea. Notable thinning toward the west, with increasing fineness of grain of material, shows that the sediment came from the east, no doubt carried by a large river from the small continental land mass (called "Appalachia") on the eastern side of North America.