6 6 Br. L. Leigh Ferinor — Isostasy, Earth quakes, Vulcanicity. 



From the isostatic point of view the chief features to emphasize 

 are: (1) that the infra-plutonic shell is everywhere a potential 

 magma-reservoir ; (2) that with relief of pressure any given portion 

 may, with expansion of volume and release of heat, become an actual 

 magma-reservoir ; (3) that the molten magma approximates in com- 

 position and therefore probably in volume much more closely to that 

 of its plutonic (i.e. non-garnetiferous) phase than to that of its 

 infra-plutonic (i.e. garnetiferous) phase. 



It will now be seen that any degree of isostatic compensation of 

 a given column of the earth's crust can be effected by the passage 

 of the requisite amount of rock from the dense, infra-plutonic, 

 garnetiferous phase to the less dense, plutonic, non-garnetiferous 

 phase, or vice versa. 



Although this hypothetical garnetiferous zone would maintain a 

 perfect hydrostatic balance in the earth's crust if the superincumbent 

 columns of rock were free to respond as a whole to every small 

 application of stress, it must be remembered tluit the upper layers of 

 the crust are under lower temperatures and pressures, and are elastic 

 rather than plastic solids, responding to applied stresses by bending 

 and fracturing instead of by flowing. They are consequently able to 

 accumulate a certain amount of strain, this ability accounting at 

 least in part for the imperfect isostatic compensation of parts of the 

 earth's crust. 



JSarthquakes are now generally regarded as the results of attempts 

 on the part of the earth's crust to release these strains by fracturing, 

 and it is suggested that every large earthquake is accompanied by 

 the passage of a proportional amount of garnetiferous into non- 

 garnetiferous rock, or vice '<'ersa, accoixling to the direction of movement 

 of the overlying rock. Although this interpretation of earthquakes 

 follows logically from the foregoing explanation of isostasy, yet it 

 was first suggested to me by Mr. R. D. Oldham in a private com- 

 munication. He writes — 



" I had been searching for some satisfactory explanation of big earthquakes, 

 which seem to require a fairly extensive though still local and fairly rapid 

 expansion in the deeper layers of the earth's crust, and had come across none 

 which had any relation to the actual composition of the earth. The passage 

 from the infra-plutonic to the plutonic mode would give just such an effect as 

 I want." 



Judging from Mr. Oldham's letter, the change of phase accom- 

 panying earthquakes is predominantly one from garnetiferous to 

 non-garnetiferous (solid or liquid). 



Perhaps before closing this note I may permit myself to indicate 

 the bearing of my hypothesis on vidcayiicity. The present general 

 opinion on the cause of vulcanicity is well expressed by Dr. Harker 

 in the following passage [Natural History of Igneous Rocks, p. 37) : — 



" It appears, then, that we must seek the immediate cause of igneous action, 

 not in the generation of heat, but chiefly in relief of pressure in certain deep- 

 seated parts of the crust where solid and molten rock are approximately in 

 thermal equilibrium . ' ' 



According to this idea the temperature of a molten lava is an 

 expression of the algebraic sum of three factors, namely: (1) the 

 temperature of its solid phase before release of pressure; (2) the latent 



