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ance in the history of the continents, and constitute the most satisfactory marks for the primary classification of geological history.

The natural geological system is 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. 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 western Minnesota to Texas. 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. The 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 Palæozoic time.