126 MR. W. S. JEVONS : REMARKS 



portion which longest remains plastic^ and that it often 

 becomes segregated or collected together into fissures 

 passing through the granite.* 



I believe that a similar action on a much larger scale is 

 the cause of all quartz reefs in Australia. Great cracks or 

 fissures are produced in the granite by some cause with 

 which we are unacquainted, passing upwards, also through 

 the overlying stratified rocks. The more liquid quartz 

 filters out of the surrounding granitic mass, carrying with 

 it the gold and the metallic sulphides, and, soon filling the 

 vacant fissure, is forced upwards and penetrates as far as 

 possible the overlying strata. We have thus an explana- 

 tion why hornblende is found in immediate connection 

 with those quartz reefs which lie amid granite ; for as the 



portion would be the first to consolidate ; and that the different varieties 

 of felspar, as well as garnets and tourmalines, being more easily liquified by 

 heat, would be the last. Precisely the reverse has taken place in the pass- 

 age of most granite aggregates from a fluid to a solid state, crystals of the 

 most fusible minerals being found enveloped in hard, transparent, glassy 

 quartz, which has often taken very faithful casts of each, so as to preserve 

 even the microscopically minute striations on the surface of prisms of tour- 

 maline. Various explanations of this phenomenon have been proposed by 

 MM. de Beaumont, Fournet, and Durocher. They I'efer to M. Guadin's 

 experiments on the fusion of quartz, which shew that silex, as it cools, has 

 the property of remaining in a viscous state, whereas alumina never does. 

 This "gelatinous flint" is supposed to retain a considerable degree of 

 plasticity long after the granitic mixture has acquired a low temperature." — 

 Lyell's Manual of Geology, fifth edition, p. 567. 



* " Yeins of pure quartz are often found in granite as in many stratified 

 rocks ; but they are not traceable, like veins of granite or trap, to large 

 bodies of rock of similar composition. They appear to have been cracks, 

 into which silicious matter was infiltered. Such segregation, as it is called, 

 can sometimes be shown to have clearly taken place long subsequently to 

 the original consolidation of the containing rock. Thus, for example, I 

 observed in the gneiss of Tronstad Strand, near Drammen in Norway, the 

 annexed section on the beach. It appears that the alternating strata of 

 whitish granitiform gneiss and black hornblende -schist were first out 

 through by a greenstone dyke, about two and a half feet wide ; then the 

 crack, a i, passed through aU these rocks, and was filled up with quartz. 

 The opposite walls of the vein are in some parts incrusted with transparent 

 crystals of quartz, the middle of the vein being filled up with common 

 opaque white quartz." — Ibid. p. 576. 



