298 FKID'IjOF NA.NSKN. M.-N. Kl. 



ficatioiis wliicli lia\c tlic siiiallcsi \oluiiic"', aiui lie points out that certain 

 rocks, bcloii^^in^ lo what he cahs the "ccloj^itc facics", which have probably 

 l)een sudileiily pressed up from \cr\- j;reat depths, are all of them very 

 heavy witli a sj)ecific .L;ra\ity of about 3.3 to 3.5, and contain heavy 

 minerals like ,t;aniet ami (hainoii'l. These rocks are unstable at the pres- 

 sure at wliich liny are now fi.und near the surface, anrl have, as it were, 

 been "i|ucnched'" b\' ihi' sudden ri'duclion of the pressure. If the pressure 

 had bicii reduced slowly, as wcaild ha\e bc-en the case if these deep-seated 

 rocks had been brouj^hi u]) towards the cririh-s surface bv gradual denud- 

 ation, they would gradually have been transformer! into rocks composed 

 of more voluminous and lighter minerals, conditioned by the existing 

 pressiu'e. 



i^skola points out that "'eclogites occupy a vohune about 15 per cent 

 smaller than that of corresponding gabbros, and the magma probablv has 

 a still larger volume. The vokmie of jadeitite (sp. gravity = 3-33) is as 

 much as 22 ])er cent smaller than that of the corrcsponfling molecular 

 mixture of albite and nephelite (sp. gravity = 2.61;". 



He also maintains that as the melting point of eclogites is raised by 

 pressure, it seems "likely that there exists a zone in the deepest parts of 

 the earth's crust where gabbroid material exists stable in the form of 

 cclogite, at temperatures under which a gabl)ro would melt if the pressure 

 were reduced". 



Eskola mentions that this conclusion was formerly rlrawn by L. I.. 

 Fermor, w ho states that eclogite must be the high pressure form of gabbro, 

 and that "we must, therefore, assume that in the infra-plutonic zone the 

 basic rocks are ])resent as eclogites and the more acid rocks as garneti- 

 ferous granites". Fermor thinks that this infra-plutonic zone may form 

 "a continuous shell round the earth, the whole of which shell is a potential 

 magma. This shell, being composed of rocks of the consistency of a plastic 

 solid, mav afford a cushion upon which the isostat'c operations of the 

 earth, believed in by some geologists, have their foundation". 



It is in my opinion obvious that if changes in volume, of the order 

 mentioned above, can be effected by changes of pressure, appreciable 

 vertical movements of the surface level of the lithosphère may be thus 

 produced. 



The magnitude of these movements would depend on the thickness 

 of the layer of those deep-seated rocks, the pressure of which is changed 

 beyond the critical point. If we suppose, for instance, that, by the denud- 

 ation of TOO metres of the continental surface (sp. g. = 2.6), the pressure 

 in a layer of deep-seated ecl(\gite (sp. g. = 3.45), 7 S nietres thick, be 

 reduced below the critical point, and this layer of rock gradually be trans- 

 formed, it may cause an upheaval of the continental surface of about 

 13 metres. 



