an average descent of five inches to the mile, is three miles per hour; that of the Mississippi, very nearly the same.) To supply such a volume 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 Delaware of about three feet of water; which, of course, is incredible, even if we suppose the floods confined to a single month of each successive year. Hence, without doubt, we may conclude that the deposition of the boulder-bearing brick clay in the Delaware valley, below Trenton, implies a depression of that region to the extent of one hundred or more feet.

Plate No. III. illustrates the general situation. The shaded portion about Trenton is the space covered by Trenton gravel in which the Palaeoliths are found. The relation of the river to the glaciated region may also be taken in at a glance.

Doubtless the region north of Trenton shared in this depression marking the Champlain period; but being above tide-water, the effects of the subsidence are not equally evident. But since, as we have seen, it proceeded at an increased rate

1 See also sections on pp. 297, 325.

towards the north and northwest, and must have greatly diminished the velocity of the torrents of that period as compared with what it would be in the present condition of things, the subsidence would aid in accounting for the terraces in the upper part of the valley.

Considering, now, this Trenton gravel, we find it to be limited at the head of tide-water to a level of about forty feet, and diminishing in height relatively to the river as you ascend the channel until, a few miles above Trenton, it merges into a terrace, which maintains a pretty uniform height of fifteen or twenty feet above the river all the way to the terminal moraine at Belvidere, sixty-five miles distant. The present descent of the valley from that point to Trenton is two hundred and thirtytwo feet, or at the rate of nearly four feet to the mile, as the river runs.

These being the conditions, it is easy to account for the accumulation of gravel at Trenton.

The transportation of gravel by a river is dependent both upon the amount of material accessible to the running stream, and upon the rapidity of the current. Towards the close of the glacial period the pebbles accessible to the running water in the upper Delaware were superabundant, having been conveniently deposited by the melting of the glacier. (The water-worn pebbles at Trenton were probably largely derived from the terminal

moraine.) But even a glacial torrent may have more loose material than it can manage, and so may silt up its bed with gravel. Hence it is not necessary to suppose the river at this point to have been of sufficient volume to fill the whole valley with water to the height of the terrace fifteen or twenty feet. The river may have flowed upon the surface of the gravel in a shallower current than the terrace would seem to imply.

But when the current passing down this declivity of three or four feet to the mile reached the level of the sea at Trenton its transporting power would be greatly diminished, and thus we should have an accumulation of gravel at the head of tide-water, without bringing into the problem of accounting for these gravels the supposition of any very extraordinary increase in the volume of the river.

The loss of transporting power upon diminishing the rapidity of a current of water is enormous. The transporting capacity of a stream of water is estimated to vary as the sixth power of the velocity, i.e. if a current is checked so that it moves at only half its former rate its transporting capacity is diminished theoretically to one sixty-fourth. is easy to see that the sudden enlargement of the valley just above Trenton, as well as the occurrence there of tide-water, would diminish the rapidity of the river, and hence cause an extraordinary

1 Le Conte's Geology, p. 18.

It

deposition of gravel when it was abundant and accessible in the glaciated region through which the river flowed.

The most likely time for this deposition to have occurred was, as we have said, near. the very close of the glacial period, when the lower moraines were fresh, and when local masses of ice still lingered in the southern valleys of the Catskills. The process of deposition must have been so rapid that it could not not have been much subsequent to the withdrawal of the continental glacier north of the Catskills. In brief, we are led to the following conclusions:

1. That the Philadelphia brick clay was deposited during the Champlain epoch, when the Delaware valley was considerably depressed below its present level.

2. Towards the close of that period, when the land had resumed nearly its present level, and the ice had nearly all disappeared south of the Catskills, the still swollen stream brought down the superabundant loose material from the kames and moraines of the upper Delaware, and deposited it in the valley below. The material was so abundant that doubtless the whole channel was silted up, so that the bed of the river was considerably above that which it now occupies. At Trenton it flowed over and through an extensive delta of coarse gravel, forty feet above its present