138 HISTORICAL GEOLOGY. 



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 cor- 

 respond 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 another, and especially where 

 there is also a striking change in the organic remains, indicates a 

 transition in the era from one set of circumstances to another, — it 

 may be a change from one level to another in the continents, a sub- 

 mergence or emergence, or some other kind of catastrophe. All 

 such transitions mark great events in the history of the continent, and 

 thus divide the era into periods, and may further subdivide the periods 

 into epochs. Hence, — 



Fifthly. Through the ages, the different continents, and often also 

 the distant regions of the same continent, had their special histories ; 

 and the periods and epochs are indicated by changes or transitions in 

 the rock-formations of the region and in their fossils. 



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

 the same in their limits, and much less in their rocks. The Devonian 

 age, for example, has a very different series of periods and epochs in 

 North America from what it has in Europe, and there is even con- 

 siderable diversity between the subdivisions in New York and the 

 Atlantic slope,- and those of the Mississippi valley. It is far from 

 certain that the commencement assigned to the Devonian in North 

 America is synchronous with that for Europe. The Carboniferous, 

 Reptilian and Mammalian ages also have their American epochs and 

 their European differing from one another ; and the differences be- 

 tween the continents increase as we come down to more modern 

 times. We add, therefore, — 



Sixthly. It is an important object in geology to ascertain as nearly 

 as possible the parallelism between the periods and epochs marked 

 off on each continent, and to study out the equivalents of the rocks, 

 each for each, that all the special histories may read as parts of one 

 general history, and thus contribute to the perfection of one geological 

 system. 



Subdivisions based on the progress of life. — In accordance with 

 the principles explained, the following subdivisions of geological time 

 are here adopted. 1 



1 The system of ages is essentially the same with that proposed by Professor 

 Agassiz, — the only difference consisting in calling the Silurian the age of Inverte- 

 brates, as suggested by Murchison, instead of considering both the Silurian and 

 Devonian the age of Fishes. 



