The late Palaeozoic witnessed mountain-making disturbances on an almost world-wide scale, extending from the middle Carboniferous to the middle of the Lower Permian. In central Europe and Spain vigorous folding took place at the end of the Lower Carboniferous, but the most important and widespread disturbances occurred in the Permian. In North America low folds were formed in the Appalachian trough from time to time all through the Palaeozoic, the evidence of which is the upheaval of the barriers described in the preceding chapters, which separated the Interior Sea from the changing bodies of water, such as the Cumberland Basin, on the east. A more energetic disturbance, with some mountain building, inaugurated the Pottsville age of the Upper Carboniferous, and this disturbance culminated at the end of the Permian in one of the greatest geographical revolutions which the history of North America has recorded. With the exception of the mountain making at the close of the Ordovician, the Palaeozoic era in North America had been a time of slow, even development, with many oscillations of level, but with few violent disturbances, and with singularly few manifestations of volcanic activity.

A little more land was added to the northern area during each period, but, so far as we can trace it, the geography of the Ordovician does not seem to have been very different from that of the Carboniferous. Throughout this long era the Appalachian geosyncline had been sinking, though with many shifts and oscillations, under an ever-increasing load of sediment, until the great trough contained a thickness of 25,000 feet or more of strata. Eventually the trough began to yield to lateral compression, and its contained strata were thrown into folds, or fractured by great overthrusts. . Thus, in place of a sinking sea-bottom along the shore of the great Interior Sea, rose the Appalachian Mountains, which in their youth may have been a very lofty range, rivalling the Alps in height. This range extends from the Hudson River to Alabama; another range from Newfoundland to Rhode Island, and a third, the Ouachita Mountains of Arkansas, are attributed to the same set of disturbances, which thus made themselves felt for a distance of 2000 miles.

Though the entire continent felt the effects of this revolution, they are less obvious in the West. On the western side of the Rocky Mountains a great unconformity is found between the Permian and Triassic members of the Red Beds. "There are reasons to suppose that this hitherto unrecognized break is widespread, and explains many discordant features of various Red-bed sections, not only in Colorado, but in the adjacent Plateau province." (Cross.) The Great Basin regiort, which had been submerged through nearly the whole Palaeozoic era, became land, and at the present time the surface rocks over most of this region are Carboniferous. It is, however, probable that the Permian has been stripped away by denudation, as it has been over nearly all of the northern plateau of Arizona.

Comparatively soon after the eastern part of the Great Basin had thus been converted into land, the ancient land area of its western border was depressed beneath the sea. It is probable that these two movements were connected, though they may have been separated by a considerable interval of time, In Nevada west of 1170 30' W. long, no Palaeozoic rocks have been found, and the Trias rests directly upon the Archaean.

However they may be explained, the geographical revolution which closed the Palaeozoic era was accompanied by the most profound and far-reaching changes which have ever occurred in the recorded history of life, after which we find ourselves in a new world. It is probable that the change was a relatively rapid one, but there are sufficient connections between the faunas and floras of the two eras to show that the later were derived from the earlier, and that the gaps are due to the imperfections of the record.