190 Correspondence — Mr. Alfred Harker. 



purpose as a particular case of contortion. In like manner the cor- 

 related stresses resolve into (i) a uniform pressure and (ii) certain 

 shearing stresses. The energy set free consists of two parts ; 

 (i) that due to compression, measured by the product of the uniform 

 pressure into the relative compression, and (ii) that due to shearing, 

 measured by the products of the shearing stresses into the amounts 

 of the corresponding shears. The total energy thus set free, except 

 in so far as it is lost by conduction of heat, must be absorbed in the 

 production of mineralogical changes. Rocks are known to be very 

 bad conductors of heat, but the amount of energy lost in this way 

 must vary with circumstances, time being an important factor. 



Again, viewing the strains and stresses in a rock-mass with 

 reference to the external forces that produce them, it is essential to 

 notice that the voluminal compression and uniform pressure depend 

 upon the sum of the forces acting in different directions (e.g. vertically 

 and horizontally) while the shears and shearing stresses depend 

 upon the differences of those forces. We may, for example, picture 

 a mass of rocks subjected to a lateral thrust and to the weight of 

 overlying rocks. If the mass be situated at no great depth, the 

 latter force may be very much less than the former, and considerable 

 shearing may be produced if the material be not a very rigid one, 

 or if the thrust be of long duration ; for shearing is, within limits, 

 proportional to the time. The pressure and the total energy set free 

 may or may not be very great, and under a comparatively small 

 cover of rocks much of the energy must be lost by conduction. It 

 is thus easy to imagine conditions under which any amount of con- 

 tortion may be produced without any metamorphism of the rocks 

 so affected. 



If the same lateral thrust operate upon a rock-mass at a greater 

 depth beneath the surface, it will be more nearly balanced by the 

 weight of the cover, and so the compression and pressure will be 

 greater, but the shears and shearing stresses less. The total energy 

 set free will be greater, and there will be less loss by conduction. 

 We may thus have metamorphism produced with or without con- 

 tortion. 



In the case of rocks at a depth, too, the time-element must be im- 

 portant. The rigidity of the mass being there materially diminished 

 — this, at least, is generally admitted — there must be a tendency to 

 propagate pressure uniformly, as in a liquid. If this property hold 

 good to any extent, shearing stresses cannot be set up unless the 

 disturbing forces increase comparatively suddenly. However this 

 may be, it appears that the contortion of rocks cannot afford an 

 accurate measure of the forces which have produced it, and that 

 contortion and dynamo-metamorphism, though due to the same 

 ultimate cause, are by no means necessarily associated in the same 

 place. One or the other phenomenon may occur alone, or both 

 together, in accordance with complex conditions, such as the depth 

 of the cover, the rigidity of the rocks affected, and the slowness or 

 rapidity of development of the disturbing forces. 



General McMahon apparently calls in question the experimental 



