24 



Geological Notes on the Khasi Bills. 



[No. 1, 



great mass of the lower series, known better as the Nahun group or 

 Lower Siwalik formation, — that I think they can be well placed on 

 that horizon. Whether this will be proved still further west, by 

 the presence of the later Mammaliferous sands and gravels of the 

 higher, and again unconformable series of the Siwalik group, is to 

 be seen, and it is a most interesting point ; or may not these last beds 

 still exist under the present plain of Sylhet and Mymensing, undis- 

 turbed, abutting like the present land surface against the lower series ? 

 The change is so sudden here, from dry sandy steep slopes to swamps, 

 that within a few paces of the hill side, the ground is covered with the 

 dead shells of Paludina and Ampullaria ; the sections seen in the beds 

 of the streams show an alternation of sands with dark clay, containing 

 the same shells. I could point out a bed, under and to the south of 

 Nahun, so precisely similar, with the above shells (particularly the more 

 lasting opercula of the latter species) that no one who had wandered 

 over both areas, examining them attentively, could fail to be struck with 

 the great similarity of their deposition. The only difference rests in 

 the present unconformity of the one, due to elevation ; and in the still 

 normal position of the other, slowly accumulating bed over bed, and 

 perhaps in some future geological age, to pass through the same mighty 

 changes. Medlicott's explanatory ideal section in the Markunda under 

 Nahun, (where also lies the beds I have just referred to) is nowhere 

 brought so forcibly to the imagination, as at the foot of these Hahiang 

 Garo Hills. 



The beds are actually at Chanda Dinga so near the perpendicular, 

 that a transition from No. 1 to No. 2 (vide Ideal Sections below) is 

 easily wrought, and this is what is actually seen at the junction near 

 Nahun, if anything greatly exaggerated in nature, from the lateral 

 force that has been introduced. 



