40 GEOLOGICAL BIOLOGY. 



lands. It is important also to note that the geological revo- 

 lution 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 accu- 

 mulating 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 geologi- 

 cal period of time above the sea-level, a great event in geo- 

 logical 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 rec- 

 ords. 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 con- 

 tinuous 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 revo- 

 lution in geological time, since the close of the Archaean, sep- 

 arates the Carboniferous from the Triassic system. In Amer- 

 ican classification, following Dana's usage, it may be called 

 the Appalachian revolution. It terminated the series of for- 

 mations 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 



