CHAPTER II.
NORTH AMERICA IN THE AGE OF REPTILES.
ITS GEOGRAPHIC AND CLIMATIC CHANGES.
North America in the Age of Reptiles would have seemed almost as strange to our eyes in its geography as in its animals and plants.

The present outlines of its coast, its mountains and valleys, its rivers and lakes, have mostly arisen since that time.

Even the more ancient parts of the continent have been profoundly modified through the incessant work of rain and rivers and of the waves, tending to wear down the land surfaces, of volcanic outbursts building them up, and of the more mysterious agencies which raise or depress vast stretches of mountain chains or even the whole area of a continent, and which tend on the whole so far as we can see, to restore or increase the relief of the continents, as the action of the surface waters tends to bring them down to or beneath the sea level.
_Alternate Overflow and Emergence of Continents._ In a broad way these agencies of elevation and of erosion have caused in their age-long struggle an alternation of periods of overflow and periods of continental emergence during geologic time.

During the periods of overflow, great portions of the low-lying parts of the continents were submerged, and formed extensive but comparatively shallow seas.

The mountains through long continued erosion were reduced to gentle and uniform slopes of comparatively slight elevation.