G. F. Wright— Unity of the Glacial Epoch. 373 



but that there were glacial periods to accompany the earlier 

 enlargements is by no means so evident. 



Futhermore, as bearing against the duality of the glacial 

 period it may be urged with great force that it is improbable 

 that two periods should so nearly duplicate one another as 

 these two are supposed to have done. To those who maintain 

 the sufficiency of Croll's astronomical cause, however, this is 

 rather an argument in favor. But on the other hand, that 

 cause would also demand a long succession of periods during 

 all the geological ages, and of these we lack sufficient proof ; 

 while it would throw the two periods which Professor Cham- 

 berlin recognizes back much farther than the facts will admit. 

 It must be said, however, that it is not wholly out of analogy 

 with known earth movements to suppose that there has been, 

 in connection with the glacial period, a succession of oscilla- 

 tions of the earth's crust nearly duplicating each other. Such 

 oscillations seem to have occurred in various geological ages, as 

 for instance during the coal period when the successive coal 

 beds were formed. And indeed, much can be said in favOr of 

 the view that such an oscillation, when once begun would per- 

 petuate itself. The loading of one part of the land surface 

 and its consequent subsidence would naturally, if not neces- 

 sarily, be attended with the elevation of other and perhaps 

 distant portions of the crust. This might secure the beginnings 

 of a glacial period in another hemisphere, which when fully 

 grown, would return the favor, and by its weight cause the 

 earth's crust to bulge out again where it had been depressed. 

 But our knowledge of these matters is too vague to reason on 

 it with any confidence, as is also that of the other causes 

 which have been suggested for the production of the phe- 

 nomena of the period. 



In conclusion it is sufficient to remark that our present 

 state of knowledge on the subject seems so imperfect that it 

 is not conducive to success in investigation to hold any theory 

 as to the unity or duality of the period with great positiveness. 

 Over-confidence on this point at the present time is likely to 

 blind the eyes of the investigator, and to hinder progress, 

 both in the collection and in the interpretation of the multitu- 

 dinous and complicated facts which everywhere invite our 

 close attention. 



