XXXIV. On the Internal Pressure to which Rock Classes may he suhjected, and 

 its possible Influence in the Production of the Laminated Structure. 

 By W. Hopkins, Esq., M.A., F.R.S., &c. 



[Read May 3, 1847-] 



One of the most curious phenomena in the constitution of rock masses, consists in the laminated 

 structure which pervades so large a portion of the older sedimentary formations, producing what 

 is called their slatv cleavage. In some cases, this lamination is comparatively coarse and ill-defined, 

 but in others (as in the roofing slates) it is so fine and regular as to leave no doubt of its being 

 the result of some kind of molecular action of the constituent particles on each other, analogous to 

 that of crystallization, and not the direct and immediate mechanical effect of external forces 

 acting on the mass. But still it would seem very possible, that these external forces may 

 maintain the mass in a state of internal constraint which may possibly be a condition favourable 

 to the production of the laminated structure, and observations have lately been made which seem 

 to afford some confirmation of this notion. Professor Phillips, some years ago, and Mr. Sharpe, 

 more I'ecently, have recognized some curious and interesting facts respecting the frequent distortion 

 of fossil shells, and other organic remains, from their original well-known forms ; and these distortions 

 appear to indicate certain relations between the positions of the cleavage planes and the directions 

 of the internal pressures which must have produced the distortions in question. These distortions 

 of determinate organic forms indicate, in fact, corresponding distortions in those elements of the 

 mass in which they are I'espectively comprized. To explain the nature of the tensions or pressures 

 acting on any such element and the distortion produced by tliem, let us denote by s a small plane 

 surface passing through any point P. Generally, there will be an action between the particles 

 (M) on one side of this small plane, and M', those in contact with them on the opposite side. 

 If s be sufficiently small, we may represent the whole action of M on M' by ps, a force having 

 a determinate direction, which we may suppose to make an angle ^ with the normal to s. Then will 



pscosS, and pssind, 



be the normal and tangential parts of the whole action of M on M', and 



— ps cos S, and — ps sin S, 



will manifestly be the same parts of the reaction of M' on M. If the normal force be a pressure, 

 it will only tend to preserve the contact of the particles immediately on opposite sides of s; but if 

 that force be a tension, then will ps cos S tend to separate these particles by motions normal to s, 

 and in opposite directions. In all cases there will be likewise forces equal to ps sin S, and 

 - ps sin ^, tending to separate any one particle immediately on one side of s, from the particle 

 originally in contact with it on the other side of s, by communicating to these particles, motions in 

 opposite directions parallel to the plane of s. If this plane assume different positions by moving 

 about P as a fixed point, the normal and tangential forces acting on it will have different values, 

 assuming maxima or minima values for certain determinate positions of s, and it is on these 

 particular positions of s that the distortion of a small portion of the mass about P, and that of any 

 organic form contained in it, will depend. Generally, The linear dimensions of the element will he 



