268 ' i Prof. Rogers's Address before the 



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

 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 whole 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 

 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 roc 

 in a succession of tremendous deluges southward across the con 



Other explanations, consisting in the main either of an union 

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

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

 ed most in accordance with the phenomena of the drift on t is 

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

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

 is to say, the origin of the smoothed surfaces and striae, the cau 

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

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

 polished and grooved surface of the whole rocky base on wind ^ 

 drift reposes, many of our geologists conceive that ice was e ^ 



the production of this phenomenon ; but some, sharing the cau ^ 

 Prof. Hitchcock, refrain from U attempting to decide whether i 



of 



