QUARTZ-rOEPHYEY. 129 



of biotite predominating over the hornblende, in my Mar examples 

 constitutes the greatest point of difference. There can be no 

 doubt, however, that although these rocks in Idar (which are in 

 the form of bosses, plugs and irregular intrusive masses) only 

 show a few cases of penological passage into the Jdar granite by 

 means of fine-grained micro-granite and granophyre, their whole 

 mineral constitution suggests that they may easily represent a 

 somewhat more hypabyssal form of the several varieties of the 

 Idar granite, although all real connection between the two pro- 

 bably lies too deeply seated to have been exposed (except rarely) 

 at the present surface of the ground. As the very similar granites 

 of Siwana and Jalor arc considered by LaTouche to be generally 

 equivalent in age to a large portion of the Malani rhyolite series 

 (he. cit. p. 25) it max- well be that the quartz-porphyries of Idar 

 are a penological connecting link between a granite of the Idar, 

 Siwana and Jalor types and the bedded Malani rhyolite flows. I 

 do not mean that the Idar porphyry was the actual vent rock of 

 the rhyolites as now exposed in AVestern Rajputana, but that they 

 constitute vents of a similar related material that further north 

 became extrusive as the acid Malani flows. 



QUARTZ VEINS. 

 Although vein-quartz lias been parenthetically mentioned in 

 connection with many of the foregoing forma- 



fea1urc" 1,01lant ^ tions > mch as thc Aravallis, Delhi Quartzite 



and Phyllite Series, it remains true that all 

 such material thus referred to is of minor importance, the veins 

 being small in thickness, irregular in position and building no special 

 surface feature. The quartz veins now about to be described 

 differ fundamentally from the above, for they have a considerable 

 thickness, rise up into well-marked and quite noticeable ridge 

 features and they continue, albeit with a broken continuity, through 

 distances of very many miles, keeping their direction steady and 

 their width normal. 



Although the known occurrences, which are separately coloured 

 on the map, keep to Aravalli country, and 



Confined to Aravalli o nerallv to that occupied by the calc-gneiss 

 oalo-gneiss. & - . . * . . 



or by the latter in combination with bosses 



of Idar granite, these veins, like so many other particular formations 



in Idar, actually show up (in every ease but one or two) as elongated 



