66 ball: geology of the rajmehal hills. 



In the northern part of the hills, close to the Simra Bungalow, 

 there is a group of small conical hills called 

 Trachytic porphyry. Gandesuri, whose outline contrasts very strikingly 

 with those of the surrounding trap and sandstones. These hills are 

 formed of a pinkish trachyte, which is porphyrytic in some places, in 

 others vesicular; occasionally the different varieties occur in a brecciated 

 mass. The opinion first, I believe, started by Dr. Buchannan, that these 

 rocks mark the position of an old crater, seems to be not improbably 

 the true view of their origin. The hills form three-fourths of a circle, 

 and may very possibly be the remnant of an ancient volcanic vent, which 

 was active during the period of the outpouring of the flows of basalt. 

 Within the circle are some white beds which may be of intertrappean 

 age, but this is not certain. Indeed, the relations of this trachyte to all 

 the surroundiug rocks is much obscured by superficial deposits, and its 

 age is by no means conclusively established. 



"Within the areas occupied by the older sedimentary rocks in the 

 Rajmehal hills, intrusive dykes, though of rare 

 Trap dykes. occurrence, are not altogether absent. Two in- 



stances at least may be mentioned : one in the section of Barakar rocks 

 south of Nargunjo described on page 32, and the other in the Dwarka 

 River section, near Deocha. In the metamorphic area, too, trap dykes 

 occur, some of which may very possibly be of the same age. Dyke-like 

 intrusions in the inter-trappean rocks are not common. The best seen 

 case is in the section of the small hill on the banks of the Ganges at 

 Sikrigali, four miles east of Sahibganj. The edges of the plant-bearing 

 shales exposed there have been disturbed by a dyke of coarsely granular 

 dark-green trap, which externally is unlike any of the bedded traps. 



Mr. Blanford, in his report on the Raniganj field, thus writes* of 

 the probable geological age of the numerous trap dykes which occur 

 in that area : " There appears good reason for supposing that these 



* Mem., Geol. Survey of India, III, p, 144. 

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