50 THE POPULAR SCIENCE MONTHLY. 



five years ; and the same writers calculate that the same amount 

 of erosion in a valley by water alone would require two and a 

 half times as long.* Other writers have made estimates less 

 favorable to ice as an agent of erosion ; but even if the amount 

 annually be but small, the cumulative effect was undoubtedly 

 very great in the case of the enormous glaciers of the Ice age. 

 The very wide areas covered with bowlder clay and drift in 

 North America, and its great average depth, have already been 

 referred to in my previous article (Popular Science Monthly, 

 April, 1894, p. 782) ; but a still more striking estimate has been 

 made of the amount of rock debris in northern Europe which can 

 be traced to Scandinavia. Dr. Amund Helland states that about 

 eight hundred thousand square miles are covered with such drift 

 to an average depth of one hundred and fifty feet, of which about 

 one hundred feet are of Scandinavian origin, the remainder being 

 local. The area of Scandinavia and Finland, from which this 

 debris has been derived, is very much less than the area over 

 which it is distributed, so that to produce it an amount equal to 

 an average thickness of two hundred and fifty-five feet must have 

 been removed from those countries. To this must be added the 

 amount which has gone into the Baltic and North Seas, and also 

 that which has been carried away by rain and rivers since the Ice 

 age passed away, and yet further, the enormous amount that still 

 remains on the lowlands of Scandinavia, and we shall then arrive 

 at an amount probably twice as great as the above estimate, that 

 is, something like five hundred feet as the average amount of ice 

 erosion of Scandinavia during the Glacial period. f Now, unless 

 this estimate is wildly and extravagantly erroneous aiid Prof. 

 Geikie adopts it as prima facie not extravagant we have an 

 amount of ice erosion so enormous as to put completely out of 

 court all the allegations of those who attempt to minimize it as a 

 mere smoothing off of sharp angles and rugged surfaces. I am 

 not aware that Prof. Bonney denies the Scandinavian origin of the 

 greater part of the northern drift, and unless he can show that its 

 quantity is something like a fiftieth part only of the estimate of 

 Dr. Helland, I can not understand how he can still maintain that 

 the glaciers and ice-sheets of the Ice age were agents of abrasion, 

 not of erosion, and that they were therefore imj)otent to grind 

 away the comparatively small amount of rock removed, under 

 the most favorable conditions, from the basins of the valley lakes 

 whose origin we are discussing. Fortnightly Review. 



[To be continued. '\ 



* Falsan, La Periode Glaciaire, p. 90. 



f Fragments of Earth Lore, by James Geikie, F. R. S., 1893, p. 167. 



