ELEMENTS OF THE GEOLOGICAL TIME-SCALE. 285 



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 con- 

 formable strata. A geological revolution is expressed by uncon- 

 formity 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 

 Archaean, 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 continu- 

 ously 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 Palaeozoic time. 



