THE POPULAR SCIENCE MONTHLY.
an opportunity offers to examine a section of virgin soil and underlying strata, as occasionally happens on the bluffs facing the river, the limit in depth of this black soil may be approximately determined. Microscopical examination of it enables one to determine the depth more accurately.
An average, derived from several such sections, leads me to infer that the depth is not over one foot, and the proportion of vegetable matter increases as the surface is approached. Of this depth of superficial soil probably not over one half has been derived from decomposition of vegetable growths. Indeed, experiment would indicate that the rotting of tree-roots yields no appreciable amount of matter. While no positive data are determinable in this matter, beyond the naked fact that rotting trees increase the bulk of top-soil, one archæological fact we do derive, which is, that the flint implements known as Indian relics belong to this superficial or "black soil," as Kahn terms it. Abundantly are they found near the surface; more sparingly the deeper we go; while below the base of this deposit of soil, at an average depth of about two feet, the argillite implements occur in greatest abundance. The accompanying diagram more clearly sets
forth the conclusions at which I have arrived, after years of careful study of this subject. By this it will be seen that, as the depth increases, the number of ordinary flint implements of Indian origin decreases; and that the reverse is true of the palæolithic implements which are a feature of the gravel-beds; and is true of that intermediate form which is characteristic of the stratum of sand capping the gravels and blending insensibly with the surface-soil. This intermediate form, which is always made of argillite, is both inand design a marked advance over the palæolithic implements, and yet is so uniform in pattern and so inferior in finish, when compared with the average flint implement of the Indian, that it has