XI. — On the Geology of the North-eastern Border of Bengal. 

 By H. T. COLEBROOKE, Esq. f.r.s. f.l.s. v.p.g.s. 



[Road January 5, 1821.] 



J. HE information which I now lay before the Society is chiefly, and almost 

 exclusively, grounded upon communications received from Mr. David Scott, 

 Commissioner at Cooch-Behar in Bengal, who has obligingly furnished spe- 

 cimens of the rocks collected by him in his visits to divers parts of the district 

 under his authority, and liberally communicated observations made by him 

 on the spot. They are valuable as the genuine remarks of an attentive 

 observer, not indeed profoundly skilled in geology, but divested of all bias of 

 preconceived opinions: and I use his communications with entire confidence. 



The specimens which have reached me were collected near the north-east 

 corner of Bengal, where more than one river of note, issuing from the bor- 

 dering mountains, enter the plain. 



The Brahm-putra, a river which meets the principal and eastern branch of 

 the Ganges at no great distance from their common junction with the sea, 

 emerges from the mountains at the north-east angle of Bengal, after a long 

 course in the Himalaya. The hilly country, which it last traverses, is Asham, 

 rarely visited and little known. Our acquaintance with its geological charac- 

 ter is confined to the mountains that give passage to the Brahm-putra, thence 

 issuing into the plain. 



On the northern bank of the river in that position is a hill at Jogigopa, 

 which is connected, not however without some breaks or intervals, with the 

 Bhotan mountains. The body of the hills consists of a large hemispherical 

 mass of gneiss. On either side of it is granite ; graphic granite on the one 

 hand, and granite of a different character and middling-sized grain on the 

 other. The last-mentioned rock is in nearly vertical masses (Mr. Scott terms 

 them strata), the direction of which is from north-east to south-west. Toward 

 the west graphic granite occurs, on the top of a large mass of gneiss. It ap- 

 pears to be divided by transverse fissures into rhomboid forms; a few cracks 

 crossing the rhombs diagonally. 



PaglanaCh is a hill on the south side of the Brahm-putra, being the north- 



