The rocks forming this earliest known land in the Western Hemisphere consist of crystalline schists, gneisses, and granite, which are considered by some geologists at least as having resulted from the metamorphism of sedimentary beds. Penetrating and intimately intermingled with these greatly altered rocks, some of them perhaps metamorphosed lavas and allied terranes, are many rocks that were forced upward from deep in the earth into fissures in a molten condition and have since cooled and crystallized. More than one epoch of metamorphism has perhaps occurred, and the entire record now accessible is exceedingly complicated. The physical conditions at the earth's surface at the close of the Archean period, as may reasonably be inferred, were not essentially different from what they are now. The land areas were eroded by streams, and the _debris_ carried to the sea and deposited, the coarser near shore and the finer farther seaward. Upward movements in the earth's crust in various places subsequently laid bare a portion of the sea-floor adjacent to the former land, and the continent was enlarged. The outline of the land as it existed previous to the upheaval which exposed this portion of the ocean's bottom would be defined by the landward margin of the material deposited. The exposed sediments would be coarsest near the former coast-line and become finer and finer seaward from it, and the fossils contained in the consolidated sands and clays would also supply evidence bearing on the origin of the rocks. It is by such interpretation of the ancient records in the light of what is now taking place that the geologist is enabled to map approximately the outline of North America at several stages in its growth in the manner shown on the series of maps here presented. Information in this connection, however, concerning both the northern and southern portions of the continent is too meagre at present to be largely utilized in these outline sketches.[5] [5] The relations of the eras referred to on these maps and the positions they occupy on the geological time-scale are shown a few pages later on a chart of the geological history of North America.