34 LECTURES 



With this brief but, I believe, sufficient account of ani- 

 mal distribution for our present purpose, we are now in 

 position to pass a step still nearer the main subject of our 

 lecture. For, without some strong hint as to the laws 

 governing the distribution of existing life, we could 

 hardly have been expected to draw a clear picture of 

 what geology has to show us for the ages that are past. 



As commonly defined in books, geology is the science 

 which treats of the physical history of the earth's crust, 

 and of the operation of those laws which are responsible 

 for the changes which have occurred in it throughout 

 time. But there is a great deal more in the science than 

 this definition would at first appear to indicate, and in 

 the writer's estimation no one among us has more beauti- 

 fully dwelt upon its real teachings than our distinguished 

 countryman, Professor Joseph Le Conte. 



Le Conte has pointed out that the study of the science 

 of geology falls naturally under three great divisional 

 heads or departments. Under structural geology we may 

 regard the earth much in the same light as the anatomist 

 regards the body, and study it from the standpoint of its 

 external form and internal structure. Or, again, under 

 dynamical geology we may have to do with what may be 

 considered the physiology of the earth, or the action upon 

 its structure by the elements, as the air, water and cer- 

 tain chemical and physical forces. Lastly, we have 

 historical geology, comparable with the science of 

 embryology for it takes into consideration the history of 

 the development of the earth and the laws which have 

 governed that development. 



Our authority, then, proceeds to show "that there are 

 certain laws underlying all development certain general 

 principles common to all history, whether of the in- 

 dividual, the race, or the earth." The geologist finds 

 that these general principles are quite as applicable to his 

 science, or to the earth's history, as they are, for 

 example, to the civic history of man, or the history of 

 civilization. 



"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- 

 formations, and rock-strata." 



