V GLACIAL EROSION OF LAKE BASINS 109 



mented to any extent from superficial sources. The boulder clays 

 are the direct products of glacial erosion. They consist essentially 

 of unweathered material. Boulders, smaller stones, grit, sand, and 

 the finer grained rock-meal or flour are all alike fresh, they have not 

 been altered chemically as they would have been had they come from 

 superficial sources. They could not have come from above, and thef/^ 

 cannot represent the weathered rock-debris of preglacial times." 

 (Earth Sculpture, i:f. 187.) 



And again (at p. 190 of the same volume) he writes: — 



" From the various facts above referred to we are justified in con- 

 cluding that glacier-ice is a most effective agent of erosion. It not 

 only abrades, rubs, smooths, and polishes, but crushes, folds, 

 disrupts, and displaces rock-masses, the amount of disturbance being 

 in proportion to the resisting jjower of the rock and the pressure 

 exerted by the ice. Other things being equal, more crushing and 

 displacement Avill be effected under a massive ice-sheet than under a 

 small valley glacier." 



Taking these various facts and arguments in connection 

 with the great number and large size of the lakes of 

 Sweden, Lapland, and Finland, almost all true rock basins, 

 the cumulative evidence in favour of the origin of such 

 lakes by glacial erosion becomes overwhelming. 



4. 2 he Alternative Thcori/ and its Difficidties. — There is 

 really only one alternative theory to that of ice-erosion for 

 the origin of the class of lakes we have been discussing, 

 viz., that they were formed a little before the glacial epoch 

 or during its continuance, by earth-movements of the 

 same nature as those which are concerned in mountain 

 formation, that is, by lateral pressure causing folds or 

 flexures of the surface ; and where such flexures occurred 

 across a valley a lake would be the result. This is Pro- 

 fessor l^onney's theory given in his paper in the Geograph- 

 ical Journal, and it is also that of Desor, Forel, Favre, and 

 other eminent continental geologists. It is explained fully 

 in the work of M. Falsan (already quoted), who also adopts 

 it; and it may be considered, therefore, that if this theory can 

 be shown to be untenable that of glacial erosion will hold 

 the field, since there is no other that can seriously compete 

 with it. Professor Bonney considers this theory completely 

 satisfactory, and he complains that the advocates of glacial 



