Page:The Journal of geology (IA journalofgeology11893univ).pdf/302
Several lesser, more or less local, revolutions have left their permanent mark in the grander structure of the rocks or in conspicuous geographical features of the restricted region of the continental area. The first of these was the Green mountain revolution which separated the (Lower Silurian) Ordovician from the (Upper Silurian) Silurian, for the eastern part of North America. The elevation, disturbance and metamorphism of the rocks of the Green Mountains stand forth as monuments of this event. The revolution is not sharply distinguishable in the rocks of the more southern or western regions. The second of these lesser revolutions is expressed most sharply in elevation and unconformity terminating the Devonian formations of Maine, New Brunswick and Nova Scotia, and may therefore be called the Acadian revolution. In the continental interior it may be indicated by the remarkable thinning out of the Devonian rocks toward the southwestward. In Tennessee, Alabama and Arkansas they are represented by a thin sheet of black shale, a few feet thick, or by but little more than a line of separation between the rocks of the Silurian below and the Carboniferous beds resting scarcely unconformably upon them. This seems to indicate an elevation of the region still further south toward the close of the Devonian, sufficient to produce extensive erosion, uncovering the Lower Silurian rocks which were again depressed to receive the marine deposits of the early Carboniferous upon their eroded surfaces.
The Appalachian revolution closed the Palæozoic time and left the great part of the eastern half of the continent above sea-level. It forms the natural interval between the Carboniferous and the overlying system, whatever that may be. Its characterteristics have already been described.
The Palisade revolution, along the eastern border of the continent, marks the division between the Jura-Triassic part of the Mesozoic time and its closing Cretaceous age. It is expressed by the trap ridges in the Connecticut valley, the Palisades and other similar tracts distributed inside the coast from Nova Scotia to North Carolina, and by the uptilting and in some cases fault-