34 MEDLICOTT^ SHILLONG PLATEAU. 



the hills. The total distinctness of the two was thus strongly- 

 implied, but not insisted upon or even asserted because the proofs 

 were not present. I can now bring forward these proofs. They tend 

 to show that the greenstone of the interior is older even than the 

 granitCj while the eruptive rock along the outer zone is comparatively 

 but of yesterday. 



As it will be convenient to have a name for this formation, I 

 would propose to call it the Silhet trap, rather than by any name 

 connected with the Shillong plateau ; for though it extends some little 

 way up the valleys, and shows nowhere in the alluvial plains, being, 

 indeed, constantly separated from them by the whole thickness of the 

 later sedimentary rocks by which it has been covered over, still its 

 affinities are altogether with the area of disturbance. Nothing can 

 be said as to its age beyond that it is very decidedly precretaceous, the 

 strata of this period resting unconformably upon it, and no intertrappean 

 deposits, nor any infratrappean younger than the metamorphics, hg,ying 

 as yet been discovered. It might be conjectured to be of the same 

 age as the trap of Rajmahal, which is considered to be Jurassic, and 

 which is its nearest neighbour on the west, at about 200 miles distance ; 

 but of this connection there is as yet no evidence. 



There are excellent sections of the Silhet trap in the three large 

 streams already mentioned. On the beautiful deep-water -reach of the 

 Umblai below Rilung this rock forms cliffs of 200 feet high rising clear 

 from the water, and displaying well the rough stratification of the 

 trap-flows ; this is also well seen in the Theria river below the 

 confluence of the Rangat stream. On the Bogapani there is the 

 best contact-section with the cretaceous sandstone. The belt of trap, 

 within the distance of 40 miles where it has been observed, preserves 

 a very constant width of two to three miles ; between the very steady 

 line on the south along the base of the sandstones and its less regular 

 boundary with the crystalline rocks on the north. The general but 



( 184 ) 



