1882 .] 
337 
[Davis. 
The question has been vigorously debated since then, but without 
much approach to an agreement, and the debate still goes on while 
each side claims a victory. 
About the same date, de Mortillet 1 proposed a similar theory 
to account for the Italian lakes, but limited the work of glacia 
ice to the sweeping out of a preglacial alluvial filling of the 
basin ; the origin of the basin itself and whether it was really a 
rock-basin or only an obstructed valley, was not discussed. 
The arguments favoring these theories are derived first, from 
the visible action of existing glaciers ; second, from the amount 
and arrangement of glacial drift ; third, from a comparison of 
the topography of glaciated and non-glaciated regions ; and 
fourth, from ignorance of any means except the supposed glacial 
erosion to produce certain observed effects. From a summary of 
these considerations , 2 we would conclude that the rate of glacial 
erosion differed little from that of the ordinary destructive 
forces acting on the same surface ; that glacial drift was in large 
part supplied by material loosened from the bed-rock by pre- 
glacial secular disintegration ; that the amount of material 
removed by ice action was not enough 'to destroy the larger 
preglacinl features of the country or to create new ones of 
great size ; that the erosion was greatest toward the centres of 
glacial dispersion where the ice acted longest, while on mar- 
ginal areas of glacial action, erosion was largely replaced by 
deposition ; that the most important effects of the glacial period 
upon drainage are the result of the concentration upon a smaller 
area of the detritus gathered from a larger area; and finally in 
more direct reference to the subject in hand, that where condi- 
tions of ice pressure and rock resistance were favorable, glacial 
1 G. de Mortillet. Note geologique sur .... le lac d’Iseo en Lombardie. Soc. Gdol. 
France, Bull, xvi, 1859, 888-905. Gastaldi and de Mortillet. Sulla escavazione 
(affouillement) dei bacini lacustri compresi negli anfiteatri morenici. Milano, Soc. 
Ital. Atti, v. 1863, 240. In 1821 Venetz wrote “ Combien n’est-il pas de petits lacs sur 
les montagnes, qui seroient pi’obablement remplis de terre et de pierres, si jadis les 
glaciers ne les avoient pas nettoyds.” Soc. Helv. Mdm., i, 2°, 1833, 33. 
2 I hope to present at an early day the detailed statements on which this sum- 
mary is based. 
PROCEEDINGS B. S. N. H. VOL. XXI. 22 JUNE, 1882, 
