1882.] 27 [Davis. 



J. D. Dana states that much of the excavation of our valleys was done 

 in the glacial period ; partly by the direct action of the glacial, but vastly 

 more by the action of subglacial streams laden with debris from the glacier. 

 (Manual of Geology, 1880, 539; also Conn. Acad. Trans. II, 1871-73, 

 41-112.) 



A. 7. Pressure Melting has been named as an important source 

 of violent, cutting currents beneath the ice ; but it involves two 

 unknown quantities, the thickness and the temperature of the gla- 

 cial sheet, and is of problematical occurrence ; if proven, it would 

 certainly account for a share of subglacial erosion, but how much 

 we cannot reckon. 



N. S. Shaler (Illustrations of the Earth's Surface ; Glaciers. 

 Boston, 1881, 146, 158 ; and these Proceedings, xvm, 1876, 126.) He also 

 suggests that the erosive action of subglacial streams may be increased by 

 their being locally produced by the greater outward conduction of the 

 earth's internal heat through some rocks than others. (On the formation of 

 the excavated Lake-JBasins of New England; these Proceedings, x, 1866, 

 358-364.) 



A. 8. Where is Ice Erosion most Effective*? The grinding 

 power of the ice itself is generally considered greatest where it is 

 thickest, so that at certain jooints local depressions will be formed, 

 which in time will become rock-basins of great depth. To a cer- 

 tain extent this is undoubtedly true, but by no means to the 

 extent claimed by the more advanced glacialists who refer to this 

 origin lakes even two thousand feet deep. For as the basin 

 deepens the motion of the ice at the bottom will be greatly 

 retarded by having to advance up hill in its escape, and the" 

 decrease of motion will soon counterbalance the increase of pres- 

 sure and so put a stop to further excavation. The velocity of 

 the under parts of a valley-glacier is very small; in a lake 

 basin it must be still farther reduced. 



J. Ball, On the formation of Alpine valleys and Alpine lakes, Phil. 

 Mag. xxv, 1863, 81; O. Fisher, as above; and Lyell, Antiquity of Man, 

 1873, 357; call attention to the retarded motion of the ice in depressions. 



B. D. Oldham has shown that a glacier could not be forced en masse with 

 uniform velocity out of a basin or up a slope by a force from behind ; it 

 would rather crush to fragments. He therefore concludes that no ©-lacier 

 ever did move through and out of a basin, overlooking the discrepancy 

 between his supposition of uniform velocity and the known motion of gla- 

 ciers. (On the Modulus of Cohesion of Ice, and its Bearing on the Theory 

 of Glacial Erosion of Lake Basins; Phil. Mag. vn, 1879, 240-247.) The 



