From the physical characteristics and composition of sand grains is it possible to decipher their origin and geological history? This is the question which the writer set himself the task of answering while endeavoring, conjointly with Prof. A. W. Grabau, to determine the origin of a certain Silurian sandrock—the Sylvania, known in outcrop and borings about the western end of Lake Erie. It may be answered that typical assemblages of the various varieties of sand grains do reveal their own geological history, and that confirmation of this history may be obtained often from a detailed study of the beds in which these grains occur. Concerning the origin of the material from which the grains were carved one may speak less confidently, but with all the obtainable data at hand it seems possible that here, too, a definite answer may be found for each particular deposit. In his . . .