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under bydroplutonic conditions act upon a magma protruded, 

 from a deep-seated rock mass containing the elements of a 

 granite. The protruded vein-rock thus becomes topazised 

 and tourmalinised. It is bardly possible to separate physi- 

 cally the moments of topazisation and final consolidation, for 

 we must conceive of this process being at work while tbe 

 vein-mass as a wbole was still viscous. Tbe pbenocrysts of 

 felspar were probably attacked and digested during their 

 passage from below. Tbe Mount Bischoff rock is essentially 

 a vein-rock, and we are disposed to refer it to the elvan group 

 as a topazised elvan rock (now topaz quartz-porphyry). It 

 has been called "eurite" in Grenville A. J. Cole's sense of 

 compact granite = quartz-felsite, quartz-porphyry, etc. Tbe 

 origiual definition of eurite by D'Aubisson de Yoisins 

 described eurite as a compact granite with dominant felspar, 

 but the name has been applied to quartz-felsites of varying 

 origin, and has long since lost its significance as a rock title. 

 Professor Cole lias sought to revive it, but it apparently has 

 not come into use in precise petrography. Eurite often 

 occurred in a rock-mass as porphyritic granite or micro-granite, 

 being the fine grained peripheral part of a body of granite, 

 while elvan is always a vein-rock. The term elvan has been 

 objected to as being somewhat indefinite, for the material of 

 the veins has consolidated sometimes as granite-porphyry, at 

 other times as felspar or quartz-porphyry. .Again, the 

 boundary line between grauite-porphvry aud quartz-porphyry 

 is not sharply defined, for though the ground-mass of the 

 former is typically microgranitic, it often passes over into the 

 compact felsitic ground-mass of quartz-porphyry. Quartz- 

 porphyry indeed is simply a modal term applied to acidic 

 rocks. Sometimes it characterises effusive sheet?, lava 

 flows, otherwise it is applied to the material of compact 

 intrusive veins (elvan). Some high authorities consider that 

 # the loose way in which " elvan " has been used ought to 

 disqualify it as a scientific term : but this does not prevent 

 us from attaching a definite meaning to the word, which we 

 do when we employ it as signifying granitoid and quartz- 

 porphyry veins extending from masses of granite into the 

 surrounding rocks. Such veins consist typically of felspar 

 and quart z-phenocrysts (accompanied by mica, hornblende, 

 or augite) in an orthoclase and quartz ground-mass, which 

 is micro-crystalline, crypto-crystalline, or felsitic. In the 

 Mt. Bischoff rock the felspar of the ground-mass has been 

 replaced by topaz. The analysis recorded by von Groddeck 

 showed no alkali, and the rock consisted practically of quartz 

 and topaz. But this would naturally be the case in parts of 

 the rock where the topazisation process had proceeded to its 

 ultimate stage. 



