CHAPTER III.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE AND THEIR TIME-VALUES.

The Systems and Geological Revolutions. The systems, although they are arbitrarily limited and classified, rep resent certain grand events in the history of the earth. Without explaining how the series of stratified rocks came to be divided into these particular ten systems, it may be said that their retention as the great units of geological classification and nomenclature is mainly due to the relatively sharp boundaries which each system exhibits in its typical locality. The systems thus serve as known and definite standards of comparison in the construction of the time-scale, as the dominance of nations, or the dominance of dynasties, in each case serves as a time-standard for the discussion of ancient human history. As the period of each dynasty in ancient history is marked by continuity in the successive steps of progress of the country, of the acts of the people and of the forms of government, and the change of dynasties is marked by a breaking of that continuity, by revolutions and readjustment of affairs, so in geological history the grand systems represent periods of continuity of deposition for the regions in which they were formed, separated from one another by grand revolutions which interrupted the regularity of deposition, and disturbed, by folding, faulting, and sometimes by metamorphosing them, the older strata upon which the succeeding strata rest unconformably and constitute the beginnings of a new system.

Geological Revolutions Local, Not Universal.-Geological revolutions were not universal for the whole earth; from which it results that these typical systems and their classification are not equally applicable to the geological formations of all

lands. It is important also to note that the geological revolution was not a sudden catastrophe, but the culmination of slowly progressing disturbances bringing the surface of the region concerned ultimately above the level of the ocean, the ocean-level being a pivotal point in geological rock formation. The area whose surface is below the sea-level may be accumulating deposits and making rocks, but so soon as the region. is lifted above the surface it becomes a region of erosion, destruction, and degradation. Whenever, therefore, in the oscillations of level, any particular part of a continental mass of the earth's crust passes permanently or for a long geological period of time above the sea-level, a great event in geological history has culminated. In case the elevation is only temporary the event is marked by unconformity, or a break in the continuity of the formations; when it is permanent, the geological record for that region ceases, except so far as fresh-water deposits in lakes may continue independent records. Hence it is that these periods of revolution are of such importance in the history of the continents, and constitute the most satisfactory marks for the primary classification of geological history.

Revolution Expressed by Unconformity and Disturbance of Strata. The natural geological system is theoretically a continuous series of conformable strata. A geological revolution. is expressed by unconformity and more or less disturbance and displacement of the strata from their original position. The grander revolutions are also recorded in the permanent elevation of mountain masses or extensive continental areas above the level of the sea, and thus out of the reach of later strata accumulation.

Appalachian Revolution. The most widely recognized revolution in geological time, since the close of the Archæan, separates the Carboniferous from the Triassic system. In American classification, following Dana's usage, it may be called the Appalachian revolution. It terminated the series of formations which, with only minor interruptions, had been continuously accumulating in the Appalachian basin from the early Cambrian period onward. It left above the sea-level not only all the Appalachian region, but the great part of the

eastern half of the continent, extending westward beyond the Mississippi River to a line running irregularly from Texas to western Minnesota. This revolution produced the Allegheny Mountains and those flexings and faultings which are still recognized in the line of lesser ridges extending from Pennsylvania to Georgia. In England, northern Europe, and northern Asia like disturbances took place at the same general period of time. In Australia, southern Africa, and South America the indications are that the revolution was not so extensive, if it took place at all at the same time. The probabilities are that while it was almost universal for the northern hemisphere, it was mainly confined to this half of the earth. The Appalachian revolution was not limited to a brief geological period, but, beginning near the close of the coal measures of the east, it did not become effective in the region of Kansas and Nebraska till the close of the Permian. wide extent of the disturbance of strata and, consequently, of records at this point in the time-scale has led to making here a primary dividing-point of the scale, marking off Paleozoic from the following Mesozoic time. Several lesser, more or less local, revolutions have left their permanent marks in the grander structure of the rocks or in conspicuous geographical features of the restricted region of the continental arca.

The

Although revolutions of the same kind, and perhaps producing greater effects upon the final condition of the crust, may have occurred previous to the deposition of the Cambrian system, as time-marks only those revolutions which occurred. after fossils appeared in the rocks, and in stratified rocks, are here noticed; and their names and the particular events recorded are those affecting the history of the North American continent.

Taconic Revolution. The first of these was the Taconic revolution, which separated the (Lower Silurian) Ordovician from the (Upper Silurian) Silurian, in the eastern part of North America. The elevation, disturbance, and metamorphism of the rocks of the Taconic mountain range along western New England, and extending from Quebec on the north to New Jersey, stand forth as monuments of this event. The Cincinnati uplift, extending from the western part of

Ontario, Canada, into Tennessee, marks a contemporaneous disturbance. Evidence of the same revolution is seen in unconformability between Ordovician and Silurian rocks in Nova Scotia and New Brunswick. The revolution is not sharply distinguishable in the rocks of the more southern or western regions.

Acadian Revolution. 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 period upon their eroded surfaces.

Appalachian Revolution. The Appalachian revolution closed the Paleozoic 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 characteristics have already been described (p. 40).

Palisade Revolution.-A revolution which affected the rocks along the eastern border of the continent during or closing the period in which the Triassic sandstones were being deposited may be called the Palisade revolution. 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 faulting of the underlying red sandstone and shale, and the resulting unconformity with the succeeding formations. The evidences of the revolution are not widely extended, nor is the time-relation of the termination of the

revolution sharply defined, but it is sufficiently so to form a natural boundary-line separating the Triassic-Jurassic from the Cretaceous. After this point of time there occurred nothing in the eastern half of the continent which deserves the name or rank of a geological revolution, except the glacial revolution which is defined further on. The western part of the continent is conspicuous for the late occurrence of its geological construction, which was chiefly after the Triassic; along the western coast the Sierra Nevada revolution marked the same general interval of time recorded by the Palisade revolution of the East. These events on the opposite borders of the continent are alike at least in preceding the Cretaceous and in terminating the formations which are of Jura-Triassic age.

Rocky Mountain Revolution.-The Rocky Mountain revolution, which resulted in the elevation and disturbance of all the rocks in the region of the Rocky Mountains, and extended from them to the border ranges, is distributed along the time from the close of the Cretaceous to the Miocene, or possibly later. It is altogether probable that the actual length of time taken in elevating, tilting, and disturbing the strata, after the last marine deposits of the pre-Laramie formations, which resulted in the permanent adding to the continent of its western third, was not longer than that consumed in the various events terminating the Paleozoic and making into permanent land the great mass of the eastern half of the continent.*

This Rocky Mountain revolution resembles the Appalachian revolution in extending over and affecting a large area of the continent, in its general upward-lifting of that area, which process extended over a long period of time, and in the great accumulation of coal or lignite which was associated with the gradual emergence of the continental mass above the sea-level. Another feature in which the two revolutions resemble each other is the wide extent of the disturb

ances recorded. The elevation of the mountain ranges, from the Pyrenees eastward to the Himalayas and to the islands

* See further regarding this revolution Dana, Manual of Geology," 4th ed., 1895, p. 875, etc., paragraph on "Post-Mesozoic Revolution: Mountainmaking and its results," also pp. 932-939.