The opinions thus set forth in the first published account of Dr. Abbott’s discoveries, have since been considerably modified, in so far as the geological age of the tool-bearing gravel of the Delaware valley is concerned. In his earlier publications he assumed, as no longer questionable, the existence of inter-glacial, if not pre-glacial, man on the continent. In his more matured views, as set forth in his _Primitive Industry_, he speaks of “having been seriously misled by the various geological reports that purport to give, in proper sequence, the respective ages of the several strata of clay, gravel, boulders, and sand, through which the river has finally worn its channel to the ocean level”;[52] so that he has probably ascribed too great an antiquity to the peculiar class of stone implements brought to light in the river-gravels of New Jersey. Dr. Abbott, accordingly, states as his more matured conclusion, confirmed by the reports of some of the most experienced geological observers, on whose judgment he relies, that the Trenton gravel, in which alone the turtle-back celts have thus far been found, is a post-glacial river deposit, made at a time when the river was larger than at present; and is the most recent of all the formations of the Delaware.[53] Here, however, the term “recent” is employed altogether relatively; and although Dr. Abbott no longer claims in the discovery of the stone implements of the gravel beds near Trenton, New Jersey, evidence of the existence of man on the American continent before the close of the Glacial period, he still refers the Trenton gravel tool-makers to an era which, at the lowest computation, precedes by thousands of years the earliest historical glimpses of Assyria, Egypt, or wherever among the most ancient nations of the Old World the beginnings of history can be traced.