lxxvi PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol. lxXlV,. 



stress in rocks, other than effects of a purely mechanical kind. It 

 is true that Sorby, in an address delivered from this Chair in 1879, 

 pointed out that an unequal distribution of ' pressure ' within a 

 rock-mass must cause differences in solubility, and illustrated the 

 consequences from the microstructure of some limestones ; but this 

 remained as one of numerous suffffestions thrown out by that 

 versatile genius, and left for others to follow up. About the same 

 time Willard Gribbs was discussing on thermodynamic principles 

 the equilibrium-relations between a liquid and a solid under stress 

 of any kind ; but his results were presented in so highly generalized 

 a form that it was long before all their implications were perceived. 



One consideration of moment to the geologist is that, when 

 pressure is applied to a mass consisting of crystals with interstitial 

 liquid, the additional pressure is borne, by the crystals and not, or 

 in much less degree, by the liquid. It follows that, whereas 

 equally distributed pressure would cause in general an elevation of 

 melting-points, unequal pressure (that is, unequal as between 

 crystals and liquid) causes always a lowering of melting-points, 

 and that of much greater amount. Johnston & Adams have 

 applied this principle to Tresca's well-known experiments on the 

 flow of metals, maintaining that the effects are due to actual 

 fusion, local and temporary, in the interstices of the crystalline 

 mass. However this may be, it cannot be doubted that the same 

 principle has an important bearing upon the fluxional deformation 

 of rocks under unequal stress, and so upon the origin of certain 

 gneissic and foliated structures. Here, of course, Ave have to do, 

 not with simple fusion, but with solution, which is subject to 

 precisely the same law. We gain a new light upon the action 

 whereby material is dissolved from those parts of a crystal or grain 

 which bear greater pressure, to be concurrently deposited upon 

 those parts where the pressure is less. This action, observed by 

 Sorby in calcareous rocks, has undoubtedly a wider application, 

 notably to the flattened and sometimes sinuous shapes assumed by 

 such a mineral as quartz in some types of crystalline schists. It 

 affords still another method by which the unequal stress set up by 

 external forces is minimized or relieved. 



There is, of course, much more than this. Even when a single 

 mineral suffers such differential solution, that which recrystallizes 

 may not be the same as that which dissolves, but a dimorphous 

 form more stable under the actual conditions. In the more general 

 case, where solution affects a number of minerals simultaneously, 

 the products which crystallize out will often be new minerals, and 



