the Mountains of Scotland. 125 
It was necessary to use substances capable of plication within 
certain limits, but incapable of supporting a greater strain without 
fracture; alternations of plaster of Paris and damp sand fulfilled 
this condition ; moreover, deformation was allowed free play by not 
weighting by the addition of any foreign body the mass of com- 
pressed beds. This was to deviate appreciably from the natural 
conditions where the weight of the beds, owing to the magnitude 
of the masses in motion, plays a considerable part, whilst in the 
experiment this action is negligeable. On the other hand, if by 
these means structures are produced comparable with those of the 
Highlands, one may conclude that the causes of the peculiarities 
presented are connected with a diminished plasticity, due either to 
the nature of the beds, or rather to their diminished thickness. 
It is, indeed, admitted now, following Professor Heim, that the 
plasticity to which the folds of mountain beds testify is due, on the 
one hand, to the slowness of the movements, and, on the other, to 
the enormous weight with which the folded parts were charged. 
For solid bodies, with sufficient pressure, the distribution of this 
pressure becomes, as with liquids, equal in every direction; and 
then, even if the cohesive forces be overcome, the particles, 
energetically maintained from all sides, are incapable of any but 
very slight relative motion; the body is deformed progressively, 
and without breaking. 
As a matter of fact, Mr. Cadell’s experiments reproduced with 
surprising fidelity some of the singular sections described in Scotland, 
notably the superposed thrust planes, and the beds piled up over 
themselves. The starting point is always the formation of a fold; 
but the beds thus brought in relief, being no longer sustained 
laterally, break, and their fragments transgress one over the other ; 
the fold remains visible in the depth, although its form has dis- 
appeared from the surface owing to this kind of breaking up into 
pieces. As regards the piling up, the experiments produce it only 
in the upper portion of the system, almost always above a thrust 
plane, and as an extreme case of the collapse of an arch. The 
friction exerted by an upper thrust plane therefore plays no part 
here, and one is led to think that it may have been so hkewise in 
the natural phenomenon, or at least that the friction above has only 
intervened so far as to cause the piled up fragments to lie in the 
direction of the movement, and to dispose them more horizontally. 
In other words, the slices of piled up beds would represent slices 
which have advanced further than that which is above them, and 
not so far as that which is beneath; and which, moreover, because 
of their small thickness, have undergone in a greater measure the 
retarding or accelerating action of the masses between which they 
lay. In this form, it will be at once perceived that the formula 
can be generalized, and gives in a very simple manner the general 
explanation of the phenomena; the thrust masses have been split up 
into a series of horizontal slices which have moved independently 
of one another; the thick masses in one piece, the others subject 
1 This is exactly the formula which I proposed two years ago, when pointing out 
