. . .Laurentian nucleus, and constituting the foundations of the present ridges of the Roeky [57] Mountains and the Alleghanies. Between these the extensive triangular area now constituting the greater part of North America, was a shallow oceanic plateau, sheltered from the cold polar currents by the Laurentian land on the north, and separated by the ridges already mentioned from the Atlantic and Pacific. It was on this great plateau of warm and sheltered ocean that what we call the Silurian fauna lived; while of the creatures that inhabited the depths of the great bounding oceans, whose abysses must have been far deeper and at a much lower temperature, we know little. During the long Silurian periods, it is true, the great American plateau underwent many revolutions, sometimes being more deeply submerged, and having clear water tenanted by vast numbers of corals and shell-fishes, at others rising so as to become shallow and to receive deposits of sand and mud; but it wa. . . Read More