Let us examine the possibilities of the case. As an initiatory premise it might be assumed, without much chance of either affirmation or denial, that the degradation of the land surface in the immediate valleys of the main streams is or has been in the past taking place at the rate of half a line per day; so far as the eye and ordinary instruments of measurement are concerned this is a quite inappreciable amount, and I see no reason why it may not be assumed as the working power of the Yukon. With this rate of erosion a valley trough or contour of about a foot and a third might be formed in the period of a single year, or of nearly seven hundred feet in five hundred years; and if we lessen the daily erosion to one quarter of the amount stated--i. e., to an eighth of a line--we should still have in this same period of five hundred years, speaking broadly, a trough of about one hundred and seventy-five feet depth, quite sufficient to have brought about most marked changes in the aspect of a drift-covered lagoon region, and perhaps ample to account for those physiognomic peculiarities which have been discovered. I am fully impressed with the magnitude of the distance which separates the amount of erosion which I have assumed--an eighth of a line daily--from the "one foot in six thousand years," which has been preached categorically from lecturn and text-book for the better part of a quarter of a century and threatens to make dogma for still another period of equal length; but the conditions here are entirely different from those of average continental denudation--in fact, have as nearly nothing in common as they can have. My observations in the tropics and subtropics have most impressively taught me the lesson of rapid changes, and with the conditions that are and have been associated with the Yukon, I am prepared for the lesson of equal change in the north. But, as a matter of fact, are we not taught of a removal in the west central United States of some twelve thousand feet of rock strata in a period not impossibly considerably less than two hundred thousand years? The one foot in sixteen years has here likewise nothing in common with the "prevailing" rate of continental destruction.