SUBDIVISIONS IN THE HISTORY. 127 



succession of rocks in one continent or part of a continent that 

 have no representatives in another. 



When an age can be proved, through careful study, to have been 

 closed by a catastrophe or a transition 'which was universal in its 

 effects, the event is accepted as a grand and striking one in geo- 

 logical history. But the proof should be obtained before the uni- 

 versality is assumed. Hence the conclusion, — 



Fourthly. The grander subdivisions or ages in geological history, 

 based on organic progress, should be laid down independently of the 

 rocks. They are universal ideas for the globe. The rocks are to be 

 divided off as nearly as practicable in accordance with them. 



Each continent, under these ages, then becomes a special study ; 

 and its history has its periods and epochs which may or may not 

 correspond in their limits with those of the other continents. Every 

 transition in the strata, as from limestone to sandstone, clay-beds, 

 or conglomerate, or from either one to the other, and especially 

 where there is also a striking change in the organic remains, indi- 

 cates a transition in the era from one set of circumstances to. an- 

 other, — it may be a change from one level to another in the conti- 

 nents, a submergence or emergence, or some other kind of catas- 

 trophe. All such transitions mark great events in the history of 

 the continent, and thus divide the era into periods, and periods 

 into epochs, and epochs, it may be, into sub-epochs. Hence, — 



Fifthly. Through the ages each continent had its special history ; 

 and the periods and epochs in that history are indicated by changes 

 or transitions in the rock-formations and their fossils. 



It is greatly to the assistance of research that some of the revolu- 

 tions of the globe have probably been nearly or quite universal. 

 The one preceding the Mammalian age appears to be an example ; 

 although, even with regard to this, further investigation is required 

 before its actual universality can be regarded as established. But 

 the periods and epochs of America and Europe are not in general 

 the same in their limits. A near cotemporaneity in rocks may be 

 proved, but not in the transitions from one rock to another. For 

 example, the Devonian age has a very different series of periods 

 and epochs in North America from what it has in Europe, and there 

 is even considerable diversity between the epochs of New York and 

 the Atlantic slope, and those of the Mississippi valley. The Car- 

 boniferous, Keptilian, and Mammalian ages also have their American 

 epochs and their European, differing from one another; and the dif- 

 ferences between the continents increase as we come down to more 

 modern times. There are Tertiary and Cretaceous rocks in America 

 as well as Europe, but there is little reason for the assumption that 



