Nor is there any certainty that any of the neolithic remains found in the newer deposits of rivers and the upper strata of caves go further, or even so far back as these relics of an Egyptian stone period. All that the evidence really shows is, that while the neolithic period must have lasted for a long time as compared with historical standards, its duration is almost infinitesimally small as compared with that of the preceding palæolithic period. Thus in Kent's Cavern neolithic remains are only found in a small surface layer of black earth from three to twelve inches thick; while below this, palæolithic implements and a quaternary fauna occur in an upper stalagmite one to three feet thick, below it in red cave earth five to six feet thick; then in a lower stalagmite in places ten or twelve feet thick, and below it again in a breccia three or four feet thick. This is confirmed by the evidence of all the caves explored in all parts of the world, which uniformly show any neolithic remains confined to a superficial layer of a few inches with many feet of palæolithic strata below them. And river-drifts in the same manner show neolithic remains confined to the alluvia and peat-beds of existing streams, while palæolithic remains occur during the whole series of deposits while these rivers were excavating their present valleys. If we say feet for inches, or twelve for one, we shall be well within the mark in estimating the comparative duration of the palæolithic and neolithic periods, as measured by the thickness of their deposits in caves and river-drifts; and as we shall see hereafter, other geological evidence from elevations and depressions, denudations and depositions, point to even a higher figure. In going back from the neolithic into the palæolithic period, we are confronted by the difficulty to which I have already referred, of there being no hard-and-fast lines by which geological eras are clearly separated from one another. Zoologically there seems to be a very decided break between the recent and the quaternary. The instances are rare and doubtful in which we can see any trace of the remains of palæolithic man, and of the fauna of extinct animals, passing gradually into those of neolithic and recent times. But geologically there is no such abrupt break. We cannot draw a line at the culmination of the last great glaciation and say, here the glacial period ends and the post-glacial begins. Nor can we say of any definite period or horizon, this is glacial and this recent.