THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 4! 



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 Alle- 

 gheny 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 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 area. 



Although revolutions of the same kind, and perhaps pro- 

 ducing 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 re- 

 corded are those affecting the history of the North American 

 continent. 



laconic Bevolution. 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 metamor- 

 phism 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 



