GEOGRAPHICAL DISTRIBUTION. 4? 



The dome-gneiss. — A striking landscape feature along this belt, 

 and one which is of great geological importance, is the frequent 

 occurrence of low, rounded hills of gneissose granitite, the " dome- 

 gneiss" of earlier workers, so named on account of its peculiar habit 

 of weathering into piles of dome-like hummocks and large, ellipsoidal 

 masses of bare rock, in which the concentric surfaces, due to exfoliation 

 by the action of sun and weather, form the most prominent divisional 

 planes. The rock is sometimes porphyritic, and has generally a 

 gneissose structure, due to parallel disposition of its inequiaxed 

 constituents ; but there is no definite banding due to alternating layers 

 of dissimilar mineral composition, such as characterises the schist 

 formation around. In mineral composition the " dome gneiss " shows 

 the typical features of a granitite (Rosenbusch), being composed of 

 quartz and microcline with smaller quantities of oligoclase, biotite, 

 hornblende and accessory sphene, apatite and zircon. The prevailing 

 colour of the felspathic constituents gives the rock a pink to purple 

 tint. Besides its mineral composition, the rock resembles undoubtedly 

 eruptive granites in the possession of autoliths formed by local concen- 

 tration of its ferromagnesian constituents, contemporaneous coarse' 

 grained veins, xenoliths of quartzite, and a well-marked zone due to 

 contact-action near its junction with the schists. These features 

 indicate an eruptive origin for the " dome-gneiss, " and account for 

 its appearance at different horizons in the schists, its occurrence 

 in large roughly lenticular bosses, as well as in thin sheets intruded 

 between the schist folia. As a consequence of this origin, its folia- 

 tion planes sometimes underlie, and sometimes appear to rest on, 

 those of the schists. Prominent and typical examples of the 

 "dome-gneiss" are exhibited in the Nero hill, 1,737 feet, west of 

 Domchlnch (24°28'; 85°42'), Banda, 1,883 ft. near Koderma (24° 28'; 

 85° 38'), Maramoko, 2,052 ft., north-east of Koderma (24°34' ; 85°43'), 

 and Banresur, 1,739ft, north-east of Gawan (24 40' ; 86V). 



The fact that the pegmatites are most abundantly developed in the 

 schists where the " dome-gneiss " is prevalent suggests a genetic rela- 

 tionship between the two, the most probable conclusion being that the 



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