GEOLOGICAL CHRONOLOGY. 



3 



basins, ridges of elevated land, and broad plateaus inter- 

 vening between the ridges, and which were at some times 

 under water, and at other times land, with many inter- 

 mediate phases. The settlement and crumpling of the 

 crust were not continuous, but took place at intervals ; 

 and each such settlement produced not only a ridging up 

 along certain lines, but also an emergence of the plains 

 or plateaus. Thus at all times there have been ridges of 

 folded rock constituting mountain-ranges, flat expansions 

 of continental plateau, sometimes dry and sometimes sub- 

 merged, and deep ocean-basins, never except in some of 

 their shallower portions elevated into land. 



By the study of the successive beds, more especially 

 of those deposited in the times of continental submer- 

 gence, we obtain a table of geological chronology which 

 expresses the several stages of the formation of the earth's 

 crust, from that early time when a solid shell first formed 

 on our nascent planet to the present day. By collecting 

 the fossil remains embedded in the several layers and 

 placing these in chronological order, we obtain in like 

 manner histories of animal and plant life parallel to the 

 physical changes indicated by the beds themselves. The 

 facts as to the sequence we obtain from the study of ex- 

 posures in cliffs, cuttings, quarries, and mines ; and by 

 correlating these local sections in a great number of places, 

 we obtain our general table of succession ; though it is to 

 be observed that in some single exposures or series of 

 exposures, like those in the great canons of Colorado, or 

 on the coasts of Great Britain, we can often in one locality 

 see nearly the whole sequence of beds. Let us observe 

 here also that, though we can trace these series of deposits 

 over the whole of the surfaces of the continents, yet if 

 the series could be seen in one spot, say in one shaft sunk 

 through the whole thickness of the earth's crust, this 

 would be sufficient for our purpose, so far as the history 

 of life is concerned. 



