It would be wrong, however, to omit to state that, though we may know at present no remains of the first dry land, we are not ignorant of its general distribution; for the present continents show, in the arrangement of their formations and mountain chains, evidence that they are parts of a plan sketched out from the beginning. It has often been remarked by physical geographers that the great lines of coast and mountain ranges are generally in directions approaching to northeast and southwest, or northwest and southeast, and that where they run in other directions, as in the case of the south of Europe and Asia, they are much broken by salient and re-entering angles, formed by lines having these directions. Professor R. Owen, of Tennessee, and Professor Pierce, of Harvard College, were, I believe, the first to point out that these lines are in reality parts of great circles tangent to the polar circles, and the latter to suggest a theory of their origin, based on the action of solar heat and the seasons on a cooling earth. This has been more fully stated by Mr. W. Lowthian Green in his curious book, "Vestiges of the Molten Globe."[85] It would appear that the great circles in question are in reality at right angles to the line of direction of the attraction of the sun and moon at the period of either solstice, and when they happen to be in conjunction or opposition at these periods; and that such circles would be the lines on which the thin crust of a cooling globe would be most likely to be ruptured by its internal tidal-wave. Whatever the cause of the phenomenon, it is evident that in the formation of its surface inequalities the earth has cracked--so to speak--along two series of great circles tangent to the polar circles; and that these, with certain subordinate lines of fracture running north and south and east and west, have determined the forms of the continents from their origin.