66 AQUEOUS AGENCIES. 



the moving force of glaciers is invalidated by the fact that he does 

 not take sufficiently into account the effect of time and sloioly -applied 

 pressure in determining shearing; and in stiffly viscous substances 

 time is the controlling element. 2. Until we understand better than 

 we now do the actual behavior of ice-molecules in glacial motion, 

 Croll's theory must be regarded only as a modification (though, per- 

 haps, an important modification) of Forbes's ; for it supposes a mo- 

 lecular differential motion determined by gravity, and into which both 

 heat and time enter as elements. It is an attempted physical explana- 

 tion of the viscosity of ice. 



Thomson's Theory. — Some time ago James Thomson brought for- 

 ward a theory which deserves far more attention than it has yet re- 

 ceived. Thomson shows that the fusing-point of ice is loiuered, and, 

 therefore, that ice at or near its fusing-point (as is the fact in glaciers) 

 is promptly melted by pressure. Now, it is obvious that, in the dif- 

 ferential motion of glaciers, whatever point at any moment receives 

 the greatest stress of pressure must melt and give way, and, the stress 

 being relieved, it must immediately again refreeze. Meantime, by 

 change of relative position of parts, the stress is transferred to some 

 other point, which in its turn melts, gives way, is refrozen, and trans- 

 fers its stress to still another point, and so on. If we compare this 

 theory with TyndalPs, in both cases the ice gives way at the point of 

 greatest stress — in the one case stress of tension, in the other of press- 

 ure — in the one case by fracture, in the other by melting. Differential 

 motion, therefore, in the one case is by fracture, change of position, 

 and regelation ; in the other by melting, change of position, and rege- 

 lation. 



Structure of Glaciers. 



There are two points connected with the structure of glaciers which 

 require notice, viz., the veined structure and the fissures. 



Veined Structure. — The ice of glaciers is not homogeneous, but con- 

 sists of white vesicular ice (white because vesicular), banded, often very 

 beautifully, with solid transparent blue ice (transparent blue because 

 solid), the banding sometimes so delicate that a hand-specimen looks 

 like striped agate. These blue veins are not continuous planes, but 

 apparently very fiat lenticular in shape, varying in thickness from a 

 line to several inches, and in length from a few inches to several feet. 

 Their direction being parallel to one another, they give a stratified or 

 cleavage structure to the glacier, and, in melting, the glacier often 

 splits or cleaves along these planes. According to Prof. Forbes, look- 

 ing upon the glacier as a whole, we may regard the strata as taking 

 the form represented by the subjoined figures. In a section parallel to 

 the surface (Fig. 56, a), the strata outcrop in the form of loops. A 

 cross-section (Fig. 5G, b) shows them lying in troughs, and a longi- 



