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PROF. S. J. SHAND ON 



[vol. Ixxii,. 



VIII. Conclusion. 



There are two explanations before us of the origin of these 

 pseudotachylytes : one is, that they represent end-products of the 

 same process as that hj which flinty crush-rocks have been pro- 

 duced ; the other, that they are true igneous intrusions. 1 have stated 

 the evidence for and against both hypotheses without prejudice. 

 I must now add that the result of my study of flinty crush-rocks 

 has been to make me chary of reasserting the normal igneous 

 origin of the pseudotachylyte. The only point as to which I am 

 really satisfied is the intrusive relation of the material to its 

 Avails. Before rejecting the igneous theory, however, on account 

 of the evidence which has already been cited against it, we may 

 pause to consider whether a melt of granite, produced within the 

 granite itself by mechanically-developed heat, would not differ in 

 its properties from a normal granitic magma owing to the absence 

 of the usual volatile constituents of a magma. Such a melt, being 

 in all probability extremely viscous, might be unable to undergo 

 differentiation or to crystallize as a magma would ; hence the 

 absence of differentiation and of phanerocrystalline products,, 

 although hardly compatible with consolidation from a magma, is, 

 perhaps, not incompatible with consolidation from a rock-melt. 

 That the pseudotachylyte is the product of such a rock-melt, as 

 distinct from a magma, is the hypothesis that best explains the 

 facts. It depends, however, on the assumption that the heat 

 necessary to fuse the granite on the large scale which the intrusions 

 indicate is capable of being produced by the sudden rupture of the 

 granite without long-continued friction or shearing. The validity 

 of this assumption, as well as the properties of such a rock-melt 

 under high pressure, can only be guessed at. Granted the necessary 

 postulates, then this hypothesis adequately explains the phenomena. 



Should the above assumption not prove acceptable, there is still 

 one possibility left. Prof. Stanislas Meunier and others still believe 

 in the causation of earthquakes by deep-seated explosions. ' The 

 conditions at Parijs are not such as Prof. Meunier postulates, but 

 there is abundant evidence that the sub-crust of these parts of 

 South Africa has in the past contained highly explosive material : 

 witness the hundreds of tuff-filled pipes which have been drilled 

 through the crust in the Orange Free State, the Transvaal, and 

 the adjacent parts of the Cape Province. The extrusion of the 

 great dolerite-dyke at Parijs itself demands an expulsive force of 

 enormous power. The form of the pseudotachylyte veins indicates 

 that the granite was shattered by a sudden gigantic impulse or 

 series of impulses. If this impulse were of the nature of an 

 explosion in the sub-crust, it would have as a necessary consequence 

 ' the outrush of incandescent gases through all the fissures of 

 the granite. In these circumstances fusion of the walls of the 

 fissures might well ensue. Sir Andrew Noble's experiment, in 

 which cordite was exploded in steel cylinders and the gases allowed 



