"The sands carried by the winds when passing over rocks sometimes _wear them smooth_, or cover them with _scratches and furrows_, as observed by W. P. Blake on granite rocks at the Pass of San Bernardino, in California. Even quartz was polished and garnets were left projecting upon pedicels of feldspar. Limestone was so much worn as to look as if the surface had been removed by solution. Similar effects have been observed by Winchell in the Grand Traverse region, Michigan. Glass in the windows of houses on Cape Cod sometimes has holes worn through it by the same means. The hint from nature has led to the use of sand, driven by a blast, with or without steam, for cutting and engraving glass, and even for cutting and carving granite and other hard rocks."[1] Gratacap describes the rock underneath the "till" as polished and oftentimes lustrous."[2] But, it may be said, if it be true that _débris_, driven by a terrible force, could have scratched and dented the rocks, could it have made long, continuous lines and grooves upon them? But the fact is, the _striæ_ on the face of the rocks covered by the Drift are _not_ continuous; [1. Dana's "Text-Book," p. 275. 2. "Popular Science Monthly," January, 1878, p. 320.] {p. 100} they do not indicate a steady and constant pressure, such as would result where a mountainous mass of ice had caught a rock and held it, as it were, in its mighty hand, and, thus holding it steadily, had scored the rocks with it as it moved forward. "The groove is of irregular depth, its floor rising and falling, as though hitches had occurred when it was first planed, the great chisel meeting resistance, or being thrown up at points along its path."[1] What other results would follow at once from contact with the comet? We have seen that, to produce the phenomena of the Glacial age, it was absolutely necessary that it must have been preceded by a period of heat, great enough to vaporize all the streams and lakes and a large part of the ocean. And we have seen that no mere ice-hypothesis gives us any clew to the cause of this. Would the comet furnish us with such heat? Let me call another witness to the stand: In the great work of Amédée Guillemin, already cited, we read: "On the other hand, it seems proved that the light of the comets is, in part, at least, borrowed from the sun. But may they not also possess a light of their own? And, on this last hypothesis, is this brightness owing to a kind of phosphorescence, or to the state of incandescence of the nucleus? Truly, if the nuclei of comets be incandescent, the smallness of their mass would eliminate from the danger of their contact with the earth only one element of destruction: _the temperature of the terrestrial atmosphere would be raised to an elevation inimical to the existence of organized beings_; and we should only escape the danger of a mechanical shock, to run into a not less frightful