GENEEAL STRATIGRAPHY. 39 



than at Singmari. Its range would seem to be connected with' the 

 area of disturbance, which was also one of subsidence. 



7. General Stratigraphical Features. 



The peculiar position of the ShiUong plateau, as an elevated area of 

 horizontal ancient sedimentary rocks, has been mentioned — that it lies 

 in an acute angle between two regions of special disturbance and of ' true 

 mountains/ On the north, on the side of the Assam valley, the 

 basal gneissic rocks of the plateau present a continuous front to the 

 Himalayas. All apparent connection of the distinctive sedimentary rocks 

 of the table-land is with the region to the east and south-east, where these 

 same groups of rocks, in immediate proximity to, and directly continu- 

 ous with, the strata of the plateau, are intensely disturbed and form 

 rugged mountain-ridges, overtopping the level of the plateau. The 

 discussion of these phenomena of disturbance would belong mainly to 

 the geology of this mountain region ; but the important features at the 

 limit of disturbance must equally be noticed in connection with the table- 

 land, and there are collateral facts to be observed in the table-land 

 area having essential bearing upon the argument. Most of these have 

 been mentioned already, but it will be well to bring them together. 



The leading stratigraphical feature of original standing in the great 

 series of unaltered sedimentary rocks is, that there was here the limit of a 

 great basin of deposition. There is throughout the whole section from 

 north to south a steady thickening of the formations, with the substitu- 

 tion of marine deposits for those that would seem to have been alluvial and 

 diluvial. This has been described in detail in noticing the cretaceous 

 strata of Cherra. The same feature was observed in all the sections to 

 the west : about Harigaon the cretaceous sandstone is banked against a 

 steep face of crystalline rocks. Prom the examination of the Silhet 

 trap it was shown that the existence of a mountainous mass of meta- 

 morphic rocks, having approximately the same southern limit as the present 

 plateau, is of very ancient standing. However different the height and 



( 189 ) 



