THE PEEPAEATION OF THE EAKTH FOR MAN's ABODE. 87 



aud afterwards lordly forest trees and the vast variety of 

 phanerogamic vegetation which now clothes and beautifies 

 the surface, for tliis period has seen all the geological and 

 biological changes that have been in progress up to the 

 present time. 



It is to this period that modern geology has almost 

 confined its attention. But geology has now taken a 

 wider view of its scope and functions, and in doing so it 

 approximates to the etymological meain'ng of the word 

 geology, as the science of the earth ; for it embraces all 

 that can be taught respecting the earth as a whole, its 

 relation to the sun, to the other planets, and to its own 

 satellite, as well as its structure and the changes to which 

 that structure is due. All terrestrial knowledge is, in fact, 

 within the scope of geological inquiry. 



It is, however, the Third Period, of which the earth has 

 preserved for our instruction copious records in the great 

 stone-book of nature, to which I must now necessarily 

 confine myself. 



From all analogy, I think it is fair to conclude that each 

 of the two previous periods had a very long duration. Lord 

 Kelvin is, liowever, of opinion that when the solidification 

 of the exterior of the globe was consummated, the surface 

 rapidly cooled and soon became fitted for the existence of 

 organic life. 



But Avhatever was the duration of the First and Second 

 Periods, there can be no hesitation in concluding that the 

 Third Period was one of prolonged duration. 



The Records of the Rocks. 



Rocks which are obviously sedimentary, or composed of 

 material worn away from other rocks — clastic rocks, as they 

 are termed by geologists — occur much lower than the 

 oldest Cambrians, for they form large masses amongst the 

 pre-Cambrian or Archeean rocks. 



The Cambrian rocks, however, have preserved the hard 

 parts of animals inhabiting the seas of the period so 

 admirably that not only the lower sub-kingdom, Hydrozoa, 

 but the much higher classes, Crustacea, Brachiopoda, and 

 Lamellibranchiata, are clearly shown to have been both in 

 existence and to have heen well developed and abundantly 

 represented. Some of these ancient rocks are con- 

 glomerates, or beds of cemented shingle, each pebble of 



