74 HATCH : THE KOLAR GOLD-FIELD. 



Notes on Roek-speeimens collected by Dr. F. H. Hateh on the Kolar 



Gold-field, 



By T. H. Holland, Officiating Superintendent, Geological Survey of India. 



The specimens sent to represent the petrology of the Kolar gold-field and 

 described in this note are grouped in accordance with the " stratigraphical " 

 order — which for want of evidence to the contrary, is taken to correspond with the 

 order of age— adopted by Mr. Foote 1 in his first description of the area, namely, 

 (i) Granitoid Gneiss, (2) Kolar schist band, (3) Intrusive basic dykes. 



In a subsequent paper, Mr. Foote 2 grouped the Kolar schist band with other 

 similarly occurring schists in South India under the name " Dharwar" system. 



(1) GRANITE-GNEISS. 



The specimens of granite-gneiss agree in general characters with the type 

 which forms the prominent tors of the Hosur and Krishnagiri taluks of the Salem 

 district. The rock is essentially hornblende-biotite granite in composition and 

 generally contains some epidote with brown pleochroic and frequently zoned 

 sphene as characteristic accessories. The biotite has frequently a green colour 

 and is occasionally altered to chlorite. The felspars include a small amount of 

 oligoclase, whilst the potash-felspars are frequently microperthitic and in the form 

 of microcline. The felspars often contain brightly polarising minute bodies of 

 presumably secondary origin. Opaque black iron-ores, apatite and zircon occur 

 as casual and irregular accessories. 



The constituents often show peripheral granulation ; but otherwise there are 

 no signs of severe crushing since consolidation, and it is unlikely that these gran- 

 ites have been seriously deformed since their solidification, which is difficult to 

 reconcile with the assumption that the associated highly folded schists are younger 

 and were laid down as sediments on the denuded surface of this granite. 



In addition to the specimens sent to represent the main granitic mass, two 

 specimens obtained near the edge of the Kolar schist band, and thus not far from 

 the visible base of the Dharwar series, are essentially granite-gneisses in compo- 

 sition. They differ, however, from the prevalent granite-gneiss in containing 

 much muscovite, in being highly crushed, in the more complete destruction of the 

 dark-coloured ferro-magnesian silicates with formation of chlorite, and in the 

 absence of sphene. So far as they go— it would of course be dangerous to 

 generalize from two specimens— these characters are in agreement with those 

 which I have described in connection with the older biotite-gneisses of South India 



l "Notes on a traverse across some gold-fields of Mysore." Rec. Geol. Surv. Ind., Vol. 

 XV (1882), p. 199. 



J " The Dharw ar System, the chief auriferous rock Series in South India." Rec. Geol. 

 Surv. Ind., Vol. XXI (iSSS), p. 40. 



