Rev. A. Irving on the Mechanics of Glaciers. 63 



rule, transported by floating ice, but that some may be the relics of 

 old moraines, that the Boulder-clay of South Brecknockshire is 

 chiefly the product of land-ice, and that the striated rock-surfaces 

 are in some cases the result of glaciers which have descended exist- 

 ing valleys. In other cases they may have been produced by an 

 ice-sheet, which it is possible may have come from the N.W. 



December 6, 1882.— J. W. Hulke, Esq., F.R.S., 

 President, in the Chair. 



The following communications were read: — 



1. Note on a Wealden Eern, Oleandridium (Tceniopteris) Bey- 

 richii, Schenk, new to Britain. Bv John E. H. Peyton, Esq., 



F.a.s. 



2. " On the Mechanics of Glaciers, more especially with relation 

 to their supposed Power of Excavation." By the Rev. A.. Irving, 

 M.A., E.G.S. 



1. The author commenced by showing that ice is comparable 

 in some respects to glass, with which (at temperatures not far 

 removed from their several points of liquefaction) it has many 

 points of physical resemblance, the chief point of difference between 

 the two bodies being the absence in ice of the great ductility which 

 characterizes glass. Ice may therefore be regarded as a near 

 approximation to the " vitreous condition " of water. 



2. The remarkable yielding property (" plasticity," " Nachgiebig- 

 keit") of ice as it exists in glaciers (which constitutes its most 

 important point of resemblance to heated glass) being recognized as 

 a fact of observation (the experiments of Tyndall and Helmholtz, 

 and the measurements of glacier-movements by the former being- 

 referred to), the deduction drawn from these facts (irrespectively of 

 the theoretical explanation of the facts themselves) in the light of 

 the simple law of conservation of energy, is that in the movement of 

 glaciers only a small residuum of " energy of motion " of the glacial 

 mass is avai]able for the work of erosion ; most of the energy is 

 expended ivithin the mass of the glacier in overcoming cohesion. 



3. If the ice, though floiving really in a way comparable to the 

 motion of a river-current (the upper layers moving faster than the 

 lower, the median portions faster than the lateral), retained its con- 

 tinuity, the strain against the rocks might be great enough to do 

 much that is required of it by the " erosion-theory ;" but here comes 

 in the remarkable absence of ductility of ice, giving birth to crevasses, 

 the varieties of which are all referable to one common principle, and 

 adverse to erosion. 



4. Prof. Tyndall was quoted as an authority for the fact that there 

 is a gradual transfer of ice-particles from the bed towards the sur- 

 face of the glacier, a fact which the author attempted to explain 

 later on in the paper by reasoning adopted from Helmholtz. The 

 fact itself is directly opposed to erosive action. 



