36 MEDLICOTT, SHILLONG PLATEAU. 



steady in its east-west course that I was kd to suppose a fault. Along 

 the stream under Soktia none but crystalline rocks and their debris 

 occur on the north bank ; occasionally in the river bed, and generally on 

 the south bank, the earthy amygdaloid is well seen, apparently with a 

 high dip towards the junction. In the bed of the stream there is 

 much debris of a dark rough basalt, with much clear oHvine, derived no 

 doubt from some bed or dyke in the trap farther up the glen. 

 Immediately above the confluence of the Soktia stream the main river 

 debouches from its narrow rock-bound gorge through the metamorphie 

 rocks into the comparatively open valley across the belt of trap. The 

 cane suspension bridge stretches from pier to pier of porphyritic gneiss 

 and gneissose quartzite, the very last mass on the north side of the 

 boundary. The actual contact is not visible ; nor can its underlie be 

 fixed ; but it must be very steep. The trap is not exposed for a score 

 of yards below the metamorphics ; there are both earthy and stony 

 varieties, but the structure cannot be clearly made out. I did not 

 observe any signs of intrusion in the rocks on either side of the junc- 

 tion here. At the very head of the Liam glen, at the confluence of 

 the head- waters, the contact is exposed : the trap here is all of the 

 compact kind ; and strong dykes of it, with transverse prismatic struc- 

 ture, traverse the main mass and seem even to penetrate the granitic rocks 

 for a short way. There is no clean-cut plane of contact ; it is irregular 

 and entangled, as it might be by the trap having flowed against a 

 rugged surface of the crystallines. 



It was in the valley west of Mamluh that I made some conclusive 

 observations upon the relation of the Silhet trap to the crystalline 

 rocks. In the saddle of the ridge to the south of the village, 

 the surface of the trap is at an elevation of about 2,800 feet over its 

 lowest observable level ; so this may be taken as a minimum thickness for 

 the formation. The ridge of the Tarna spur is about 310 feet lower than 

 the top of the trap, this upper portion being entirely composed of the 



( 186 ) 



