[ 3' 3 



Vindhyans here is probably due to the depression in the 

 main axis of the basin. The Lower Vindhyan rocks of the 

 Son valley consist of limestones, shale, sandstone, shaley 

 sandstone, trappoid beds, porcellanic shales, and con- 

 glomeratic and calcareous sandstone. True metamorphic 

 rocks, viz.j gneiss and granitoids, encroach upon the 

 zone of the transition rocks in Bihar where for some 

 miles north of the Grand Trunk Road, west of Gaya, 

 gneiss reaches quite across the strike of the slates. Several 

 hills isolated on the alluvial plain in this neighbourhood 

 are of thorough granite. Immediately east of Gaya transi- 

 tion rocks appear again on the prolongation of those on 

 the Son valley and having the same strike. They form 

 several groups of hills in East Bihar, known as the Maher, 

 Rajagriha, Shaikhpura, and Gidhour hills, which stand clear 

 of the main gneissic area and more or less isolated in the 

 alluvial plains, and those of Mohabar and Bhiaura on the 

 northern margin of the gneissic upland. All these isolated 

 Bihar rocks belong to one system, massive quartzites appear- 

 ing on the sides of the hills and the associated schists or 

 slates appearing obscurely in the valleys. On the north side 

 of the Bhiaura ridge the bottom quartzites lie steeply against 

 the " dome gneiss " as the peculiar rounded and poised 

 masses of gneiss are called. Elsewhere schistose gneiss 

 occurs at the boundary. True granitic intrusion may be 

 observed in the soft earthy schists. In the neighbourhood 

 of Gaya many forms of special metamorphism and of contact 

 action are well exhibited. At Lukhisarai the quartzite rests 

 against an amorphous mass of pseudo-crystalline granitoid 

 rock of much less sharply defined texture than at Shaikhpura 

 in which strings of pebbles can be detected. This amorphous 

 mass rests on beds of coarse conglomerate. Another outcrop 

 of conglomeratic schist appears in the east end of the Gid- 

 hour range. 



33. The gneissic uplands of Hazaribagh in Chutia Nagpur, 



