MU. HOPKINS ON THE THEOET OF THE MOTION OF OLACIEES. 
709 
I have proved that they must in all cases differ by an angle of 45°. I have thought it 
right to say thus much to vindicate my claim to having been the first to give any expla- 
nation, founded on true mechanical principles, of the phenomena in question. 
32. That the theoretical law above enunciated with respect to the directions of trans- 
verse crevasses is the actual law, is determined, I conceive, beyond all doubt. It was, 
in fact, so distinctly recognized by M. Agassiz and others *, before careful observations 
had been made on glacial motion, that it was regarded as a proof that the sides of a 
glacier moved faster than its central portion. But the best and, I believe, the only 
accurate recorded evidence on the subject is that afforded by the admirable map 
pubhshed by M. Agassiz, of the glacier of the Aar. There the crevasses, in a particular 
locality, are laid down with geometrical accuracy, and the eye recognizes at once the 
law in question, notwithstanding the minor deviations which must necessarily result in 
such a case from local and irregular causes f. Principal Foebes, however, does not 
seem to recognize this law; for he observes J that he agrees with some preceding 
observers in believing crevasses to be mere accidents of glacial motion. I have, however, 
demonstrated, as I had done before the publication of the paper in which this opinion 
is expressed, that though a glacier should be affected by no irregularities of surface 
along its margins or bed tending to dislocate it, the internal tensions arising from the 
more rapid motion of its central portions would always tend to produce oblique trans- 
verse marginal fissures. Local and irregular causes may frequently counterbalance this 
tendency ; but the cause itself which produces these fissures is no more accidental than 
the law of the motion in which it originates. It might as well be asserted that the 
longitudinal or radiating crevasses of the lower part of the glacier of the Phone were 
merely accidental phenomena, — an assertion which few glacialists, I imagine, would 
venture to maintain. 
33. The positions of the crevasses as above determined, will be those in which they 
are originally formed. It is manifest that transverse crevasses will change their angular 
positions with reference to the axis of the glacier, by the more rapid motion of its axial 
portion. They will thus become more approximately perpendicular to the axis. More- 
over, as they deviate from their original positions, less force will be exerted to keep them 
open, and to counteract any independent or local causes tending to close them. They 
do thus finally disappear, and generally at points not remote from those where they 
were originally formed, and where local conditions probably aided their formation. 
A closing up of the crevasses after a certain time, presents no ditficulty ; but the entire 
* Systeme Glaciaire, p. 305. 
t I cannot here omit the expression of my conviction of the inadequate justice which has been rendered 
in this country to the ‘ Systeme Glaciaire’ of M. Agassiz. It professes to be, as it is, principally descriptive, 
and contains more accurate details on many points than any other work professes to offer. The map which 
accompanies it, founded on the very careful trigonometrical survey of M. Wild, is full of beautifully deli- 
neated details. We possess no other glacial document of the kind at all comparable to it. I shall have 
other occasions to refer to it. 
f Occasional Papers, p. 151. 
MDCCCLXII. 5 E 
