38 GIlOLOGICAL NOTES ON ASSAM. 



While noticing the rocks of Cherrapoonjee^ I would not omit a eurioUS 

 p ,. , phenomenon of denudation which is there exhibited, 



of denudation, j^ ^g ^j^g feature that strikes one most on first 



aj)proaching the station^ and it puzzled me for some time to find an expla- 

 nation of it. The little rounded hillocks^ whether isolated or in groups, 

 scattered over the plateau of Cherra station, form a striking contrast to 

 the prevailing scarped character of the main ridges. They are evidentlj, 

 on the whole, homogeneous in composition and without any definite 

 structure, — no partial scarp or projecting ridge betrays the presence of a 

 continuous mass of rock in any definite position. On the road about 



Mawsmai these hillocks are ciit at several places, showing them to be 



• 

 composed, to their very base, of a loose brecciated accumulation of broken 



rock materials, large angular blocks of sandstone in a base of more commi- 

 nuted matter. At first I could see nothing for it but glacial action ; I 

 thought these hillocks must be remnants of moraines, although this 

 opinion was a good deal shaken by my not being able to find any debris 

 but that of rocks known to occur above the Cherra sandstone. I am 

 now convinced that these little mounds are the result of a very difierent 

 process of denudation, namely, the wholesale removal by the solvent 

 action of subterranean water, of the band of nummulitic limestone which 

 once covered this area. Mr. Oldham has described (p. 136) the actual 

 operation of this process in the excavations of caves in the limestone of tlie 

 plateau to south-west of Cherra, and in the falling in of large areas of the 

 overlying rocks where the limestone had been thus removed. The pre- 

 sent position of some of these rounded hillocks of broken rock away from 

 the limestone scarp, and close to that of the great gorge, proves beyond a 

 question in what manner the plateau of Cherra has been cleared. These 

 little hills of broken material could not for a moment have resisted coast- 

 action. It is but one more fact to the evidence that abounds in this 

 region of the prodigious results accomplished by atmospheric action 

 ( 424 ) 



