64 Geological Society: — 



5. The instance of the Morteratsch glacier was more particularly 

 considered ; and an attempt was made to show that the assumptions 

 which underlie the reasoning by which Prof. Tyndall has endea- 

 voured to meet the objections which have been raised to the erosion- 

 theory from observations of the Morteratsch, are incompatible with 

 sound mechanical principles. 



6. The important law of the lowering of the freezing-point of 

 water by pressure was next discussed, and reasoning adopted from 

 Helmholtz, which leads to the remarkable conclusion that within 

 the glacier water at 0° C. exists in contact with ice below 0° C. 

 This was accepted by the author as the explanation of the otherwise 

 unintelligible fact referred to in 4. 



7. The last point led to the discussion of Dr. CrolPs views on 

 glacier-movement. The author gave reasons for rejecting Dr. CrolPs 

 so-called " molecular theory " of the movement of glaciers (it is 

 really little more than a restatement of the regelation theory, dis- 

 guised by a misuse of the terms "molecule" and "molecular"), 

 and for not sharing his feeling of "mystery" about the theory of 

 regelation. 



8. A real work of erosion was shown to go on in connexion with 

 glaciers, by the direct action of the glacier-streams ; but the same 

 objections apply to these as to streams flowing in an open valley, as 

 agents capable of excavating basin-like hollows. 



9. The remainder of the paper was mainly occupied with a consi- 

 deration of " tarns " among the mountains. Here it was admitted 

 that a glacier may work in a different manner from a glacier moving- 

 down a valley; and so it was thought many small rock-basins 

 (now tarns) may have been formed at the foot of precipices. 

 On the other hand it was maintained that many tarns occupy 

 hollows formed by earth-movements on the mountain-slopes, or by 

 moraines. 



10. In conclusion the author strengthened his position by point- 

 ing to the rejection of the erosion- theory by such high authorities as 

 Professors Bonney, Helmholtz, and Credner, and Mr. John Ball, 

 and expressed his regret at finding himself at issue with Sir A. 

 Ramsay, to whose geological writings we all owe so much. 



Generally, the author concluded, from mechanical and physical 

 considerations, that far too much erosive power has been attributed 

 by some writers to glaciers, and that it is doubtful if the work of 

 actual excavation has been accomplished by them at all. The dif- 

 ferential movement of glaciers he attributed to three causes : — 

 (1) cracking and regelation (Tyndall and Helmholtz) ; (2) genera- 

 tion of heat by friction within the glacier (Helmholtz) ; (3) the 

 penetration of the glacier by luminous solar energy, the absorption 

 of this by opaque bodies contained in the ice (stones, earth, organic 

 germs, &c), and the transformation of it in this way into heat. To 

 this last he attributed the greater differential movement of the 

 glacier (a) by day than by night, (b) in summer than in winter. 



