Page:Popular Science Monthly Volume 12.djvu/341

Rh  have been smoothed away by the same agency which has planed the rocks everywhere. Only the roches moutonnées have been left furrowed and scratched upon one side, whence the abrading and engraving tool advanced, but vipon the other unscored, and hidden beneath a tail of fragments ground from their opposite slopes. The significance of this we shall see later.

Thus, imperfectly described, we have reviewed the most prominent features of a comparatively modern period, viz., the widely-grooved and polished condition of northern rocks, especially hard-grained rocks, which retain these impressions; the occurrence of wandering bowlders, transported longer or shorter distances from their primitive sites, and the detrital matter from continental abrasion deeply burying the rocky face of the country, and in ridges, mounds, and sheets, extending east and west, and along the greater water-courses, stretching itself down into the Southern States in irregular tails and projections. We will now venture to examine the theories advanced to explain these singular phenomena, and describe that one which best accounts for these facts, with many correlated ones, offering an hypothesis which rationally secures their complete and harmonious agreement.



