The Rev. Dr. Irving — Dynamic Metamorphism. 563 



the relative value of the two modifying (in the case of a deep-seated 

 rock-mass) the whole metataxic result. Mr. Harker applies the 

 idea in some criticisms of a paper of General MacMahon's on the 

 " Crumpled Culm-measures of Bude " (See Geol. Mag. April, 1890). 



Suppose a case in which the two forces were equal or nearly so. 

 As long as this condition of things continued, the rock-mass would 

 not shear at all ; it would be in the condition which Professor Heim 

 calls one of "latent plasticity," if no lateral movement in any 

 direction were possible. This is equivalent to hydrostatic pressure, 

 as I have pointed out (op. cit. pp. 47, 48). Compression of the 

 mass might result, but not shearing. Until the limit of compressi- 

 bility was reached, there must be internal friction, with the necessary 

 thermal effect. As. however, the superincumbent mass became 

 diminished by denudation at the surface (the lateral force continuing 

 undiminished, or even increasing, from one or more causes), shear- 

 ing in the direction of cleavage-dip would set in : friction again, 

 with its thermal eifect. The quantity of heat generated might be 

 very large ; but its wide distribution in time and space would 

 certainly give us, in most cases, insignificant results as regards 

 intensity of heat (temperature). Op. cit. p. 54. Even the low 

 coefficient of conductivity of rock-materials would probably suffice 

 to make dissipation of the heat-energy keep pace with its production; 

 so that it may be questioned if a rise of temperature of any practical 

 value, above that at which the rock-mass was maintained by the 

 internal heat of the earth (or the proximity of a highly-heated mass 

 of igneous rock), would ensue. While granting therefore that under 

 conditions of intense compression (op. cit. p. 55) crystallization 

 might be facilitated for chemical bodies already formed, it may be 

 questioned whether the distinction, drawn by Mr. Harker for deep- 

 seated masses, is a very real one. 



The same question arises in connexion with the Eev. 0. Fisher's 

 treatment of the subject in the July Number of this Magazine; and 

 it yet remains to be shown that any considerable rise of temperature 

 (sufficient to induce chemical change) can be postulated, or that 

 much, if anything, beyond those changes which I have called " meta- 

 tropic " (op. cit.), can be accounted for by dynamic action, except 

 in rapid movements along "shear-planes" (op. cit. pp. 124-126). 



Mr. Fisher suggests (p. 304) that the mechanical energy of the 

 lateral pressure is resolved in part into the " molecular form of 

 chemical action." What can this mean? If it means that new 

 chemical compounds are formed, it is not molecular, but atomic 

 energy, which is brought into play. We surely cannot suppose 

 (indeed, we cannot, without rolling back the development of chemical 

 theory a few decades at least, suppose) that chemical combination 

 is the mere addition of one element or lower compound to another; 

 that is to say, that the configuration of the resultant compound 

 is the mere sum of the configurations of the antecedent molecules. 

 It is something much more complex than that. But chemical 

 combination must generate heat, the mechanical effect of which is 

 to expand the mass, and so tend to discount the direct effect of 



