﻿On the Composition and Constitution of certain Cornish Rocks. 87 



the views of Mr. Croll, if he admits, as I think all must admit, 

 that the temperature of the interior of the glacier is constant ; 

 and especially when it is coupled with the other fact, that a gla- 

 cier continues to advance in winter, even at times when, owing 

 to the absence of a snow covering, the temperature of the su- 

 perficial stratum is considerably below the freezing-point. 



Admitting that other causes, and especially the infiltration 

 of surface-water, may have a small share in the result, I think 

 the main point to be considered is this : the condition of the 

 glacier is such that, if we consider the superficial stratum to be 

 absolutely neutral — or 3 in other words, that the forces acting on it 

 are such as to make it move exactly at the same rate as the next 

 underlying portion — a certain not inconsiderable rate of motion 

 is attained. In such a state of things let a change take place 

 in the condition of the superficial stratum by which the forces 

 that resist its onward movement are very much diminished ; it 

 is obvious that by its weight and by the cohesion existing be- 

 tween it and the remainder of the mass, this will tend to drag 

 the lower portion forward. As far as we can judge of the nature 

 of plastic bodies, it would appear that the resistance which they 

 offer to change of form diminishes in a ratio far more rapid than 

 the increase of pressure. Their molecular condition is in a state 

 of unstable equilibrium : at a given moment certain pressures 

 are balanced by certain resistances ; but a slight increment of 

 pressure may overcome the resistance and cause change in the 

 relative position of the particles. This, as I believe, is the true 

 explanation of the effect of warm weather on advancing glaciers ; 

 and the effect is aided by the fact that through open crevasses 

 the heat often penetrates partially into the interior. Conversely, 

 winter cold increases to a slight extent the resistance to the on- 

 ward motion of a glacier, by making the superficial stratum 

 rather less ready to yield than the interior of the glacier, so that 

 the rate of advance is then reduced somewhat below what may be 

 termed its normal velocity, namely that which it would obtain if 

 the influence of the superficial stratum were absolutely removed. 



XIV. On the Chemical Composition and Microscopic Constitution 

 of certain Cornish Rocks. By J. Arthur Phillips, F.C.S., 

 M. Inst. C.E., #c* 



THE County of Cornwall forms a peninsula principally con- 

 sisting of " killas," a name applied by Cornish miners 

 to all sedimentary cleavable rocks. It is difficult to determine 

 the geological age of these, further than that they belong to the 

 palaeozoic era ; and it is believed, from the rare and imperfectly 

 * Communicated by the Author. 



