SILHET TEAPa 35 



decided unconformability with the cretaceous formation has been best 

 seen in the section of the Theria river. It is very perceptible above 

 the river on the east, viewed from the cliffs south of Cherra Punji : the 

 southerly fall of the sandstones becomes more marked from a point above 

 the north boundary of the trap, in the angle of the cliffs above 

 Soktia j and the axis of the great flexure would run inside the south 

 boundary of the trap ; still in the middle of the zone of trap a 

 northerly inclination of the flows is very apparent. The same is better 

 observed in the bed of the river itself : The separate trap-flows 

 are well weathered out and they have a northerly dip of 10° to 15°. 

 It is only in the middle of . the belt of trap that this arrangement is 

 discernible. For some distance near either boundary the trap is quite 

 amorphous ; but this is, apparently, due to obliteration of the structure 

 by crushing in the neighbourhood of the harder rocks, for the com- 

 position of the trap, at least near the southern boundary, is on the 

 whole the same as elsewhere, but the different varieties, — earthy, 

 amygdaloidal, and compact — seem all jumbled up together. It is, 

 perhaps, possible that'the disturbance noticed in the Silhet trap may 

 have been produced at the same time and by the same causes as the 

 great disturbance of the overlying sedimentary series; and of course 

 this is partly the case, as in the crushing at the south boundary ; but 

 I rather think that the diametrically opposite direction of the dip 

 suggests an independent, prior, period of disturbance; one probably 

 connected with a depression of the area of the Shillong plateau, which, 

 it would seem, was defined as a mountain area, with approximately its 

 present base, previous to the out-pouring of the trap. 



The opiaion I have just expressed is based upon the nature of the 

 junction of the trap with the metamorphic rocks. I first examiued this 

 junction in the glens of Liam and Soktia. The system of secondary 

 valleys to which these belong is manifestly determined by the junction 

 of the trap. In those two glens the line of contact is so remarkably 



( 185 ) 



