QUARTZ VEINS. 13 i 



Quartzite in a more or less ilregtilar and incomplete way, there 

 is no mistaking the long chain of veins in the western part of Idar 

 for anything but a pure vein rock of some order. 



Another feature is the brecciation that they frequently show, speci- 



. H men No. 4 ^ (12617, PI. 15, fig. 5) and S- 9 



other features. \iAOlt}, 1 L 10, ng. 0) are examples of this. Under 



the microscope also, there are certain characteris- 

 tics attending the structure of these veins that are only seen in them. 

 The silica in crystallising has not as a rule imitated the even, 

 granulitic appearance of that in the Delhi Quartzite, but appears 

 more in the form of radiating and parallel fibrous groupings of chalce- 

 donic ^micro-crystals, specimen No. ffo (1251 (5).' Occasionally, as in 

 No. Ijk (12516) from summit of small hill I mile N.E. of Rupal, 

 blurred felspar shapes appear and the grating structure of microcline 

 (impossible to mistake) in one instance is faithfully represented 

 (12517). This seems to suggest that these veins may be regarded 

 as an ultra-acid differentiation product of some acid magma, such 

 as that of the Jdar granite, or else that actual granite or other acid 

 igneous rock has been silicified in situ. I regret that these rocks 

 have not as yet been studied as closely as they deserve, for equally 

 well-developed ultra-acid reefs of this nature are well-known in 

 many parts of the Archaean of India. 



BASIC DYKES. 



In contrast with the acid division of younger intrusive igneous 



Feeble development. rocks ' wMch we havc scen to be *«■ re- 

 presented iii Idar State, there are no corre- 

 sponding igneous masses of an intermediate composition, for instance 

 no syenites and no diorites, whilst there arc only a very few 

 feebly developed examples of hypabyssal. or plutonic basic, 

 character. They make an entirely insignificant show on the map 

 and are generally incapable of representation, only one or two 

 being sufficiently large to be outlined on the I inch =^1 mile map 

 in separate colour. 



The first of these that I shall refer to is found along one line 



Outcrops between of strike cropping out at 3 or 1 places above 



Kawi. Bnihma aml the alhl ™™ (which are probably all connected) 



namely, 2} 2 miles S.byW. of Khed Brahma, 

 \ mile N.E. of Rera, and one larger and wider outcrop forming part 

 of the hill at Kawa. 



