GEOLOnrCAL NOTES ON ASSAM. 55 



rivers. Clear grey sand g-rcaily predominates in the silt of the Bramah- 

 pootra, exactly like that composing the Sub-Himalayan sandstones. This 

 character of the deposits from this great river is traceal)le too far down 

 in the delta. In coming from Sylhet to Koostea the change on both 

 sides is most marked between the brown earthy deposits of the Soorma 

 and the Ganges,, and the clear sandy sediments of the Bramahpootra. 

 In the older alluvium on the north side of the Assam valley^ the same 

 may be observed; the high bank on which Suddya stands is a mass of 

 partially consolidated sand, yet even here this dominant character 

 does not completely exclude other influences ; the high land of Bishnath 

 and of Tezpoor is composed for the upper six or eight feet of strono- 

 oehreous sandy clay, resting on pebbly sand. On the south of the 

 valley the sandy element is subordinate, but the deposits of the older 

 alluvium are unlike the silt of the rivers. The section at Santok Mookh 

 on the Dikhoo near Nazeerah will illustrate the whole case. At Santok 

 Mookh the Dikhoo just overcuts the surface of a deposit of small boul- 

 ders ; all the stones, as in parallel positions in the rivers to the east, are 

 of the fine hard lower Sevalik type of sandstone. The actual river de- 

 posit over this is chiefly a fine dark-grey sand, largely compounded of 

 small slaty grains. The whole is in irregular beds of various thickness 

 varying in colour and consistence with the proportion of clay, some 

 layers being* pure clay. At the base of this deposit there are many lo^s of 

 wood not showing am' signs of carbonization. A ferruginous ooze is 

 common at the contact mth the boulder bed. At the bend immediately 

 below the Mookh a totally different deposit shows in the high right 

 bank against which the river turns. The bottom, twentj' feet or so, is a 

 mass of stiff" blue clay ; several layers in it are shaly and largely carbo- 

 naceous. The small logs in this clay are highly carbonized, almost to 

 the condition of lignite, hard, shiny black, fracture sharp, subconchoidal. 

 The top of this clay is darker than the rest, and it is capped by eight 

 feet of strong slightly oehreous uularainated clay, the surface being four 



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