MAN AXD THE GLACIAL PERIOD. 635 



very nearly the same.) To supply such a yolunie of water 

 as this, the whole accumulation of ice in the upper Delaware 

 would suffice for only five hundred days, or for about sixteen 

 months. And to furnish this amount of water there would 

 need to be, during such floods, a daily accumulation by rains 

 and the melting ice over the whole upper valley of the Dela- 

 ware of about three feet of water, which of course is incred- 

 ible, eyen if we suppose the floods confined to a single month 

 of each successiye year. Hence, without doubt, we may con- 

 clude that the deposition of the bowlder-bearing brick-clay 

 in the Delaware Valley below Trenton implies a depression 

 of that region to the extent of one hundred or more feet. 



Doubtless the region north of Trenton shared in this de- 

 pression, but, being aboye the tide-water, the effects would 

 not be equally evident. The valley above Trenton is narrow; 

 at Lambertville, about twelve miles up the stream, a trap- 

 dike contracts the valley to a width of about one quarter 

 of a mile. Above this point the supposition of floods suffi- 

 cient to deposit the bowlder-bearing clay is, therefore, not in- 

 credible, especially since the descent in the stream was prob- 

 ably less then than now. For the depression of that period 

 proceeded, as we have seen, at increased rate northward. 

 In Montreal, it was five hundred feet ; in Vermont, about 

 three hundred feet ; and how much more or less in the 

 vicinity of Lake Erie we can not tell. Such depression 

 would greatly diminish the velocity of the torrent, and the 

 narrow places in the valley would work to the same end. 

 Professor Dana has shown that in the lower part of the val- 

 ley of the Connecticut Eiver the floods rose during the Cham- 

 plain epoch from one hundred and fifty to two hundred feet 

 above the present high-water mark. But the Connecticut 

 Eiver Valley below Middletown is contracted by trap-dikes 

 much as the Delaware is at Lambertville ; and the drain- 

 age basin of the Connecticut is three times as extensive as 

 that of the Delaware (being twenty thousand square miles). 

 The effect of this obstruction, however, is partly offset by 

 the branch currents which, as Professor Dana shows, set 

 off from the Connecticut at various places above Middle- 

 town. 



