25 i M7\ 0. Fisher, Clh. — Glacial Erosion of Lake Basins. 



degree questioning the correctness of Mr. Bonney's observations, or of 

 his direct conclusions from them, it still seems that the possibility of 

 the erosion of a great lake by a glacier has not been disproved. For 

 the assumption made by Professor Eamsay of the immense extension 

 of the ice, brings the case under wholly different conditions from any 

 which can be now observed, because the present Swiss glaciers do not 

 extend over such localities as would be favourable to the excavation 

 of lakes, and, even if they did so, observations made at their edges 

 could tell us little of what may have gone on beneath the central 

 parts of a considerable ice-field. 



It is satisfactory to find that Mr. Bonney gives the weight of his 

 authority to uphold Forbes's opinion that ice is plastic. There can 

 be no doubt that, when sufficient masses are under consideration, 

 and sufficient time is allowed, it affects on a large scale the move- 

 ments of a liquid under the action of gravity. Thus, if two portions 

 of glacier ice of unequal height were placed side by side in a con- 

 taining hollow, the more lofty mass would have its lower parts 

 pressed out laterally into the lower parts of the other, raising its 

 surface until the two become sensibly of the same height. Hence 

 we see two tributary branches of a glacier unite to form one trunk 

 glacier of a uniform level, whose surface obeys the same laws of 

 form as that of a river, swelling slightly above the general plane 

 where the supply of material is more rapid, and the motion greater. 

 Hence also a glacier fills successively the narrower and wider 

 portions of its bed. But this accommodation of the form to the 

 requirements of gravity is a slow process ; as is shown by the ice- 

 walls, exposed for some distance where the glacier has lately passed 

 a projecting cape of rock ; for at such a place the ice has already a 

 longitudinal motion ; while the transverse motion, in virtue of 

 which it eventually fills the wider channel, has to be set up de novo 

 after it has passed the narrow. 



This is the source of the so-called vis a tergo. For the tendency 

 of the ice, descending its rocky channel, is to heap itself up at any 

 point, until, becoming too heavy to be supported by the rigidity of 

 its parts and the friction of its rocky bed, and pressing outwards in 

 all directions, it is constrained to move forwards, and, if there be 

 liberty to do so, also sideways, since it cannot force back the weight 

 behind it. 



Let us suppose that, under the influence of this pressure, the end 

 of the glacier is expanded over a comparatively plane country. 

 And perhaps the simplest mode of considering whether it could 

 excavate a lake-basin will be to consider a basin already existing, 

 and to inquire whether it would be competent to deepen it. For if 

 it could do that, no doubt it could originate it. 



My object then in writing this is to refer to what seems to me 

 a point of great importance which I put forward in my second 

 letter to the Reader of April 9, 1864, and which I do not re- 

 member to have seen adverted to in any of the discussions on 

 this question. If ice were to fill a lake-basin, it must necessarily 

 be under hydrostatic pressure ; that is, there must be a stratum 



