268 Prof. Rogers^s Address before the 



ted before the epoch of the mastodon and megatherium. Of the lime 

 occupied in the formation of the drift, we have not data even for con- 

 jecture. It must have been immense indeed, if icebergs were the prin- 

 cipal agents of dispersion ; nor could it have been brief, even if pro- 

 duced by a succession of paroxysmal disturbances. Yet the whole pe- 

 riod constitutes, as it were, but a single beat of that slow-swinging pen- 

 dulum which has counted the innumerable successive stages in the ge- 

 ological history of our globe. 



The principal hypotheses proposed for explaining the detrital phe- 

 nomena, are — 



First, the theory which attributes the scratches on the rocky floor of 

 the drift, and the dispersion of the far-carried fragmentary materials, 

 to the agency of ice, creeping forward with a slow velocity but an enor- 

 mous momentum, like the glaciers of the Alps, grinding down and fine- 

 ly grooving the jagged asperities of the surface, and bearing on its 

 back the collected rubbish in the mountain slopes, and strewing this still 

 further by a rapid thaw : 



Secondly, the theory which imputes the wl^^ to icebergs, loaded 

 with detrital matter, and floating southward until stranded on the sur- 

 face of the submerged land, which the ice-fields are conceived to have 

 smoothed and scored through the agency of innumerable fragments 

 frozen into their lower surfaces : 



Thirdly, the theory which supposes no general permanent submer- 

 sion of the land, but imagines one or more paroxysmal movements of 

 the earth's crust in the higher northern latitudes to have sent a portion 

 of the contents of the Arctic seas — water, ice, and fragmentary rock — 

 in a succession of tremendous deluges southward across the continent. 



Other explanations, consisting in the main either of an union or of 

 modifications of the chief features of these hypotheses, have also been 

 suggested and find advocates. Which of these doctrines is to be deem- 

 ed most in accordance with the phenomena of the drift on this conti- 

 nent, is a point which still causes considerable diversity of opinion, and 

 discussion is still busy in relation to each branch of the problem, that 

 is to say, the origin of the smoothed surfaces and strise, the cause of 

 the wide dispersion of the erratics, the source of the currents, and the 

 condition of level of the land. Upon the question of the origin of the 

 polished and grooved surface of the whole rocky base on which the 

 drift reposes, many of our geologists conceive that ice was essential to 

 the production of this phenomenon ; but some, sharing the caution of 

 Prof. Hitchcock, refrain from " attempting to decide whether it has 



