40 MEDLICOTT^ SHILLONG PLATEAU. 



form of this mass may have been at the period of the Silhet trap^ it had 

 assumed approximately its present form along a very wide margin before 

 the cretaceous period, as is shown by the distribution of the outliers of 

 the cretaceous strata ; so that the materials for these deposits, which were 

 probably largely derived from that quarter, must have come from a con- 

 siderable distance further north. The culminating line of the crystal- 

 line area then may have been over the present area of middle and lower 

 Assam, or may even have joined on to the Himalayas. The changes of 

 level that took place during the disturbance that produced such ranges 

 as the Burrail, at a time well advanced in the tertiary period, may well 

 have led to the extensive erosion of the Assam vaUey. There is some 

 evidence for this in the distribution of the drainage : in the eastern 

 portion of the plateau, the rivers flow northwards right across it from 

 the Bm-rail, or rather from the edge of the area of disturbance. 



The one main stratigraphical featm*e of origin subsequent to the 

 formation of the unaltered sedimentary series is the continuous semi- 

 anticlinal flexure on the east and south boundary, along which all the strata 

 bend down into the area of disturbance. This structural phenomenon 

 indicates, at least f Timet, facie, that the initial movement within the area 

 of disturbance was one of subsidence. The age of this break-up has 

 not yet been fixed, for want of fossil evidence ; but it would seem to 

 have been well advanced in the tertiary period, for there is a very 

 great thickness of supra-nummulitie deposits, which have been accumu- 

 lated in unbroken parallelism upon the cretaceous beds of the plateau, 

 and which maintain that parallelism where all are highly inclined. Some 

 local appearances of unconformability have been described, and many 

 more such may be observed, without shaking the main fact that no 

 general contorting force acted upon this great sedimentary series until 

 after the deposition of its latest member. 



It will be at once perceived what a striking example the conditions 

 of this area afford of the theory of M. M. Herschell and Babbage 

 ( 190 ) 



