Dr. L. Le igh Fermor — Isostasy, Earthquakes, Vulcanicity. 65 



III. — The Relationship of Isostast, Earthquakes, and Vulcanicity 

 TO THE Earth's Infra-Plutonic Shell. 



By L. Leigh Fermor, D.Sc, A.E.S.M., F.G.S., 

 Geological Survey of India. 



rilHE recent discussion on the origin of the Himalaya, initiated by 

 1 Colonel S. G. Biirrard in his paper " On the Origin of the 

 Himalaya Mountains"/ has centred in the theories of isostasy and 

 mountain compensation advocated by the Rev. 0. Eislier and 

 Mr. J. r. Hayford. 



In his latest contribution to the subject Colonel Burrard " states 

 very clearly the problem of isostasy now requiring solution — 



" Continents and mountains have been found to be compensated by under- 

 lying deficiencies of density ; how has this condition resembling hydrostatic 

 equilibrium arisen upon a solid globe of rock ? ' ' 



In a recent paper ^ I have advanced reasons for believing in the 

 existence in the earth's crust at a certain depth, at present unknown, 

 of a highly garnetiferous shell of rock, for which the name infra- 

 plutonic zone or shell is suggested. This zone is situated at sucli 

 a depth that pressure becomes a dominant factor in mineral trans- 

 formations, the accompanying high temperature ensuring a sufficient 

 degree of molecular mobility. It is suggested that under the influence 

 of these high pressures (and temperatures) reactions will ensue 

 between the various ferromagnesian silicates — micas, amphiboles, 

 pyroxenes, olivines — and anoi'thite felspar, with the formation of 

 garnet as a characteristic mineral : the reason assigned is that 

 a reduction of volume accompanied by an absorption of heat is 

 thereby effected, it being accepted that the garnet-forming reactions 

 are endotliermic. 



If one can accept the philosophical necessity for the existence of 

 this infra-plutonic shell, then some theory of isostasy seems logically 

 to follow, with the garnetiferous plastic-solid shell as the cushion 

 upon which the isostatic adjustments of the earth's crust have their 

 foundation. 



For if by tectonic movement the superincumbent pressure be 

 relieved over any section of the infra-plutonic shell, the prevailing 

 high temperature will cause the infra-plutonic rock to liquefy, with 

 considerable increase of volume and evolution of heat (the resultant 

 melt corresponding presumably no longer to the infra-plutonic pliase, 

 but to the silicates into which it will freeze on solidification under 

 reduced pressure). This highly heated magma is now available for 

 intrusion to higher levels in the crust according to the direction of 

 application of dynamic pressure, and it may either solidify at higher 

 levels under reduced pressures as a plutonic rock, or it may reacli the 

 earth's surface as a lava, either directly, without chemical modifica- 

 tion, but, probably, more often only after suffering some degree of 

 magmatic differentiation in intermediate reservoirs. 



^ Survey of India, Professional Paper No. 12, 1912. 



2 Geol. Mag., September, 1913, pp. 385-8. 



•' " Preliminary Note on Garnet as a Geological Barometer and on an Infra- 

 Plutonic Zone in the Earth's Crust": Kecords Geol. Surv. Ind., xliii, 

 pp. 41-7, 1913. 



DECADE VI. — VOL. I. — NO. II. 5 



