the glacial period; it is gorged in spring-time with ice-floes formed within its own area and swept into it by its great affluent, as was the ancient Delaware throughout a longer and more rigorous winter; and its bottom is a submerged terrace-plain of loam, sand, and gravel differing from the Trenton gravel only in the less proportion of ice-borne materials within it. In the shoal Susquehanna-Chesapeake estuary grow a great variety of aquatic plants, harboring multitudes of minute animals, which together furnish abundant food for fish and water-fowl, and just as it is now among white men a far-famed fishing and hunting ground, so it was a notable resort of the aborigines, as attested by the village sites about its shores; and since its shallow waters may be waded over half its area and the simplest water-craft outlives the low billows of its storms, the primitive spear-head and stone sinker doubtless underlie the cartridge-shell and leaden sinker of the present, just as the "turtle-back" of Trenton underlies the finely chipped flint of the surface. During the later ice epoch of the Quaternary the climate of the Delaware estuary was less tolerable than that of the present Chesapeake estuary, but other conditions were more favorable to concentration of piscine, avian, and human life within and about it. There the river-breeding fishes were stopped in their instinctive ascent toward former spawning-grounds to increase their kind; there the migratory birds must have ended their vernal journeyings to nest and hatch; there also the flora, forced southward before the advancing ice, must have grown mixed and varied; there the land fauna, pressed by the northern cold and attracted by the forage and carnage, must have lingered and multiplied; and there primitive men must have congregated and dominated over all. It is true but not surprising that the fragile remains of fish, fowl, plants, or even human bones have never been found in the porous and thoroughly leached Trenton gravels, associated with the implements and the more massive bones of mastodon, bison, and reindeer; but the locality was as distant from the ice-front as the arctic breeding-grounds of to-day, and moderately mild climate is attested by the wonderfully abundant implements and the numerous population they represent.
The artificial origin of the "turtle-backs" has been questioned, and their abundance has been regarded as proof of their natural origin; and it is therefore not a work of supererogation to point out that the Trenton gravels are largely wrought for railway ballast, and have been scanned by the thousand tons by eager workmen with the hope of reward before their eyes, and to repeat that the argillite of which the implements are fashioned rarely occurs in the deposit in the form of natural pebbles. Of any hundred bits of argillite selected at random from the gravel-