TRANSACTIONS OF THE SECTIONS. 73) 
there appear to be several systems of these planes of structure, the positions of which 
would seem to bear determinate relations to each other, it would appear extremely 
difficult to account for the phenomena by the simple operation of pressure alone. It 
is likely, I conceive, that more complicated causes have been at work, though press- 
ure may have exercised an important influence. Again, it has been suggested that 
what has been termed the force of shrinkage, or that internal tension which may be 
produced in extended masses by their contraction from the loss of heat or moisture, 
might be sufficient to account for the formation of joints, the positions of which, you 
will recollect, approximate more or less to verticality. But there is one curious feature 
in these phenomena, which appears to me inexplicable on this theory. It is well 
known that in conglomerate formations in which large boulders are imbedded, the 
joints pass completely through the boulders without any apparent interruption, 
According to this theory, these boulders must have been pulled in two by the force 
of shrinkage. It is in this that the difficulty I allude to consists; for it would appear, 
I think, extremely difficult to conceive how the general matrix of such a conglo- 
merate, wnatever may be the force with which it contracts, should obtain a sufficiently 
powerful grasp on the two opposite halves of a smooth and rounded boulder, a few 
inches in diameter, to pull them asunder. I suspect in all these phenomena the 
working of some agencies more refined than those of simple compression and extension, 
The subject of the motion of glaciers is one of interest to geologists; for unless we 
understand the causes of such motion, it will be impossible for us to assign to former 
glaciers their proper degree of efficiency in the transport of erratic blocks, and to di- 
stinguish between the effects of glacial and of floating ice, and those of powerfuljeur- 
rents. An important step has recently been made in this subject by the application 
of a discovery made by Mr. Faraday a few years ago, that if one lump of ice be laid 
upon another, the contiguous surfaces being sufficiently smooth to ensure perfect con- 
tact, the two pieces in a short time will become firmly frozen together into one con- 
tinuous transparent mass, although the temperature of the atmosphere in which they 
are placed be many degrees above the freezing temperature. Dr. Tyndall has the 
merit of applying this fact to the explanation of certain glacial phenomena. There 
are two recognized ways in which the motion of a glacier takes place—one by the 
sliding of the whole glacial mass over the bed of the valley in which it exists, and the 
other by the whole mass changing its form in consequence of the pressures and ten- 
sions to which it is subjected. The former mode of progression is that recognized by 
the sliding theory; the second is that recognized by what has been termed the viscous 
theory of Prof. Forbes. The viscous theory appeared to be generally recognized. 
Still, to many persons it seemed difficult to reconcile the property of viscosity with 
the fragility and apparent inflexibility and inextensibility of ice itself. On the other 
hand, if this property of viscosity, or something of the kind, were denied, how could 
we account for the fact of the different fragments into which a glacier is frequently 
broken, becoming again united into one continuous mass? Dr. Tyndall has, I con- 
ceive, solved the difficulty. Glacial ice, unlike a viscous mass, will bear very little 
extension. It breaks and cracks suddenly; but the separate pieces, when subsequently 
squeezed together, again become by regelation (as it is termed) one continuous mass. 
After some general remarks on the cause of the laminar structure of glaciers, 
he remarked that there was no doubt Dr. Tyndall was right in supposing the lamine 
of blue and white ice to be perpendicular to the directions of maximum pressure, 
He reminded the Section, that he had himself shown this fourteen years ago, by a 
rigorous mathematical investigation of the internal pressures and tensions of the 
glacial mass, resulting from the more rapid motion in its central than in its lateral 
portions ; and the results of this investigation were entirely sanctioned by the experi- 
mental elucidations which Dr. Tyndall had given of the nature of these internal press- 
ures. The physical explanation of the formation of the veins could scarcely yet be 
regarded as complete; but the actual perpendicularity of the lamin of ice to the 
directions of maximum pressure within a glacier, and the probable perpendicularity 
to those directions of the laminz in rock masses of laminated structure, would seem 
to establish some relation between these structures in rocks and glacial ice, giving an 
interest to this peculiar structure in the latter case, which it might not otherwise ap- 
pear to possess for one who should regard it merely as a geologist. 
He concluded his remarks by referring to the papers to be read before the Section, 
