PART III. 

 HISTORICAL GEOLOGY; 



OR, THE HISTORY OF THE EVOLUTION' OF EARTH-STRUCTURE 

 AND OF THE ORGANIC KINGDOM. 



CHAPTER I. 

 GENERAL PRINCIPLES. 



Geology is the history of the evolution of earth-forms and earth- 

 inhabitants. There are certain laws underlying all development — cer- 

 tain general principles common to all history, whether of the indi- 

 vidual, the race, or the earth. We wish to illustrate these general prin- 

 ciples in the more unfamiliar field of geology by running a parallel 

 between the history of the earth and other more familiar forms of 

 history. 



1. All history is divided into eras, ages, periods, epochs, separated 

 from each other more or less trenchantly by great events producing 

 great changes. In written history these are treated, according to their 

 importance, in separate volumes, or separate chapters, sections, etc. 

 So earth-history is similarly divided into geological eras, ages, periods, 

 etc. ; and these have been recorded by Nature in separate rock- systems, 

 rock-series, rock-groups, and rock- formations. In geology these terms, 

 both those referring to divisions of time and those referring to divisions 

 of record, are unfortunately loosely and interchangeably used. We 

 shall strive to use them as definitely as possible, the eras and the cor- 

 responding rock-systems being the primary divisions, and the others 

 subdivisions in the order mentioned. 



2. In all history successive eras, ages, periods, etc., usually graduate 

 insensibly into each other, though sometimes the change is more rapid 

 and revolutionary. In individual history childhood usually graduates 

 into youth, and youth into manhood; yet sometimes a remarkable 

 event determines a more rapid change. In social and political life, too, 

 successive phases of civilization embodying successive dominant prin- 

 ciples usually graduate into each other ; yet great events have some- 



