Fortunately for science, Dr. C. C. Abbott, whose tastes for archæological investigations were early developed, had his residence upon the border of this glacial delta terrace at Trenton, and as early as 1875 began to find rough-stone implements of a peculiar type in the talus of the bank where the river was undermining the terrace. In turning his attention to the numerous fresh exposures of gravel made by railroad and other excavations during the following year, he found several of the implements in undisturbed strata, some of which were sixteen feet below the surface. Since that time he has continued to make discoveries at various intervals. In 1888 he had found four hundred implements of the palæolithic type at Trenton, sixty of which had been taken from recorded depths in the gravel, two hundred and fifty from the talus at the bluff facing the river, and the remainder from the surface, or derived from collectors who did not record the positions or circumstances under which they were found. [Illustration: Fig. 65.--Section across the Delaware River at Trenton. New Jersey: _a_, _a_, Philadelphia red gravel and brick-clay (McGee's Columbia deposit); _b_. _b_, Trenton gravel, in which the implements are found: _c_, present flood-plain of the Delaware River (after Lewis). (From Abbott's Primitive Industry.)] The material from which the implements at Trenton are made is argillite--that is, a clay slate which has been so metamorphosed as to be susceptible of fracture, almost like flint. It is, however, by no means capable of being worked into such delicate forms as flint is. But as it is the only material in the vicinity capable of being chipped, prehistoric men of that vicinity were compelled to make a virtue of necessity and use the inferior material. Of all the implements found by Dr. Abbott in the gravel, only one was flint; while upon the surface innumerable arrow-heads of flint have been found. The transition, also, in the type of implements is as sudden as that in the kind of material of which they are made. Below the superficial deposit of black soil, extending down to the depth of about one foot, the modern Indian flint implements entirely disappear, and implements of palæolithic type only are found. [Illustration: Fig. 66.--Section of the Trenton gravel in which the implements described in the text are found. The shelf on which the man stands is made in process of excavation. The gravel is the same above and below (photograph by Abbott).] [Illustration: Fig. 67.--Face view of argillite implement, found by Dr. C. C. Abbott, in 1876, at Trenton, New Jersey, in gravel, three feet from face of bluff, and twenty-two feet from the surface (No. 10,985) (Putnam).]