ON THE STRUCTURE AND MOTION OF GLACIERS 3 
surface of a sluggish river. In short, the analogies are put forth so 
clearly, so ably, and so persistently, that it is not surprising that this 
theory stands at present without a competitor. The phenomena, 
indeed, are really such as to render it difficult to abstain from forming 
some such opinion as to their cause. The resemblance of many 
glaciers to ‘a pail of thickish mortar poured out;” the gradual 
changing of a straight line transverse to the glacier into a curve, in 
consequence of the swifter motion of the centre; the bent grooves 
upon the surface ; the disposition of the dirt; the contortions of the 
ice, a specimen of which, as sketched near the Heisseplatte upon the 
Lower Grindelwald glacier, is given in fig. 1, and of which other 
striking examples have been adduced by M. Escher, in proof of the 
plasticity of the substance,—are all calculated to establish the convic- 
tion, that the mass must be either viscous, or endowed with some other 
2 
MMM Me 
property mechanically equivalent to viscosity. The question then occurs, 
is the viscosity real or apparent? Does any property equivalent to 
viscosity exist, in virtue of which ice can move and mould itself in the 
manner indicated, and which is still in harmony with our experience 
of the non-viscous character of the substance? If such a property can 
be shown to exist, the choice will rest between a quality which ice is 
proved to possess, and one which, in opposition to general experience, 
it is assumed to possess, in accounting for a series of phenomena 
which either the real or the hypothetical property might be sufficient 
to produce. In the next section, the existence of a true cause will be 
pointed out, which reconciles the properties of ice, exhibited even by 
hand specimens, with the apparent evidences of viscosity already 
referred to, and which, though it has been overlooked hitherto, must 
play a part of the highest importance in the phenomena of the glacier 
world. 
B 2 
