PEOFESSOE TYIS'DALL ON THE VEINED STEHCTIJEE OF GLACIEES. 301 
architecture. The conclusion that they were squeezed flat seems to have been drawn by 
M. Agassiz, and reproduced by subsequent writers, without due regard to the difficulties 
associated with it. That the pressures of. a glacier are so parcelled out as to squeeze 
contiguous fragments of ice, not exceeding a cubic inch in size, in all possible directions, 
is so improbable, that reflection alone must throw great difficulties in the way of its 
acceptance. 
It is with some diffidence that I here venture to express an opinion upon a question 
that I have not specially examined ; but it appears to me probable that the decomposi- 
tion of glacier ice into large granules, regarding which so much has been written, may 
be connected with the foregoing facts. The ice of glaciers is sometimes disintegrated 
to a great depth; causing it to resemble an aggregate of jointed polyhedra more than a 
coherent solid. I was very near losing my life in 1857 on the Col du Geant by trusting 
to such ice; and last summer I found vast masses of it at the end of the Allalein 
glacier. Blocks a cubic yard and upwards in volume, fell to pieces to their very 
centres on being overturned ; they were an aggregate of granules, whose average volume 
scarcely exceeded a cubic inch. From the constitution which the foregoing observations 
assign to glacier ice, this disintegration seems natural. The substance is composed of 
fragments which are wrtually crystallized in difierent planes; and it is not to be expected 
that the union along the surfaces, though they may be invisible when the ice is sound, 
is as intimate as that among the difierent parts of a mass homogeneously crystallized! 
Besides, ice no doubt, and all uniaxal crystals, expands by an augmentation of tempera- 
ture, differently in different directions, and hence a differential motion of the particles 
on both sides of one of the above surfaces when the volume of the substance is changed 
by heat or cold is unavoidable. Such surfaces then would become surfaces of discon- 
tinuity, and perhaps produce that granular condition which has occupied so much of the 
attention of observers. 
§ 10. Physical Analysis of the Veined Structure. 
The relation of pressure and structure has been shown in the foregoing pages, but 
the mode in which the pressure acts remains yet to be considered. As regards their 
causes, slaty cleavage and slaty structure have been reduced to one and the same ; but 
as regards the operation of that cause, no two things can, I imagine, in some respects at 
least, be more different. 
In a note at page 336 of the ‘Proceedings of the Boyal Society’ for January 1857, I 
refer to an experiment in which a clear mass of ice was caused by pressure to resemble 
a piece of fissured gypsum, and I there promised the full details of the experiment in' 
due time. In my paper on the Physical Properties of Ice this promise is fulfilled ; I 
have shown how a mass of compact ice may be liquefied by pressure, in parallel planes 
perpendicular to the direction of the force, and explained the effect by reference to 
the ingenious deductions of Mr. James Thomsox from Carxot’s maxim. 
Let the attention now be fixed on the state of a glacier at the base of one of the 
MDCCCLIX. 9 „ 
