REFERENCE TO THE ANTIQUITY OF MAX. 



405 



belief, for I placed the oldest of the old valley- gravels at the com- 

 mencement of this Postglacial period, whereas I now think it pro- 

 bable that in the south of England and in France many of them 

 may date back to full Glacial times. I could never, however, agree 

 to the great length of time assigned to the postglacial period. The 

 adoption of a rate of denudation based on that of the present day 

 always seemed to me open to grave objections, and in this belief all 

 subsequent experience has confirmed me. Dr. Croll, who, with others, 

 adopts the generally accepted rate of denudation, namely, one foot 

 of rock or soil removed off the general level of the country during 

 6000 Years, nevertheless remarks " if the rate of denudation be at 

 present so great, what must it have been during the Glacial epoch ? 

 It must have been something enormous." This led him, it is true, 

 to reject the alternative date of from 980,000 to 720,000 years ago 

 for the Glacial epoch, and to adopt the one terminating 80,000 years 

 ago ; yet much of his argument is based on the assumption of the 

 above-named rate of denudation *. 



But it is no more possible to judge of the rate of denudation during 

 the Glacial period by that of river-action at the present day than it 

 was to estimate the rate of flow of the Greenlaud ice by Alpine experi- 

 ence. The enormous pressure and wear of ice from 2000 to 6000 feet 

 thick in contracted valley-channels, especially in fiords, where, as for 

 example in Greenland, it stood from 1800 to 2000 feet higher than 

 now ; the powerful disintegrating effects of extreme cold on rocks ; the 

 annual action of ground-ice in rivers, and of the sweeping and devas- 

 tating floods, resulting from the melting of the winter's snow and 

 surplus ice, combined to produce results of which it is impossible 

 to judge by the ordinary work of these temperate latitudes. We 

 must go to high northern latitudes to find any terms of comparison. 



I am unable at present to go more fully into this subject, but I would 

 just allude to some interesting corroborative testimony recently brought 

 forward by Prof. J. D. Dana in connexion with the phenomena of the 

 Connecticut valley f . The numerous old river-terraces in this valley 

 extend for a distance of 250 miles inland. The river has excavated 

 a valley through the ancient high-level plain to a depth of from 

 1-50 to 200 feet, with a width of from one eighth of a mile to one mile. 

 The mean depth of the river in flood at this Postglacial (Champlain) 

 period is estimated by Dana to have been about 140 feet t, the mean 

 height of the present floods being about 26 feet. The mean width 

 of the upper section of the flooded stream he estimates at 6000 feet. 

 Taking these measures, together with the mean slope, he obtains a 

 maximum velocity of over twelve miles an hour, with a mean of about 

 three or four miles, whence some idea may be formed of the enormous 

 transporting power of the river of that period. The annual rainfall in 



* Though little change has yet been made in the line of argument, there has 

 been a growing belief amongst geologists that the present rate of change has 

 not always been uniform, and must not be taken as the measure for all past and 

 all future time. See some pregnant observations on this point by Dr. Archibald 

 Geikie in the Trans. Geol. Soc. Glasgow for March 1868, p. 188. 



t American Journ. of Science for March 1882. 



X This seems to me possibly too extreme a depth. 



