78 SMITH: GEOLOGY OF MIKIR HILLS IN ASSAM. 



I found a vein of very curious dark trap — the 'coal ' of report— about 

 20 feet wide, running east and west in the gneiss. The trap was a 

 remarkable, coarse-grained rock shewing a distinct ophitic structure. 

 Under the microscope it is seen to be composed of light-green to 

 colourless hornblendes, with large brown biotite crystals ophitic 

 round them. It contains also some plagioclase felspar, and a few 

 quartz grains, with a fibrous mineral of high double refraction — 

 apparently an altered hornblende, 



Cretaceous. 



Between the gneiss and the definite nummulitic band, some doubt- 

 ful beds of intermediate age occur, here dealt with as cretaceous. 



The only occurrence of pre-tertiary sedimentary rocks, in the 

 south Mikir hills, is on the flanks of Longloi hill. The peak of 

 the hill is of gneiss, lapped round by sedimentary rocks. Thick 

 jungle obscures everything, however, and it was only with the help 

 of the neighbouring villagers that any rock exposures could be found, 

 About one mile west-south-west of Bihar the following section 

 is seen in a small nala, which traverses the 



Longloi hill. 



outcrop of the beds. The section is presumably 

 in regular descending order, approaching the gneissic core of the hill. 

 The first exposure of rock seen in the nala is a barrier of 15 feet 

 of scft earthy, sandy limestone, full of nummulites, dipping 6o°N.W. 

 For 100 yards above this barrier no exposure is seen, except where 

 a small landslip shows a few yards of soft, sandy clay of recent 

 appearance. Then, with no trace of overlying beds, the following 

 section is found, dipping io° to 20 N.N.W., in descending order : — 



Feet. Inch. 

 Brownish-black coal, with many grains of fossil resin, slightly 



shaly at the base ......... 9 4 



Similar brownish-black coal with specks of resin ... 2 8 



Purple to black carbonaceous shale 4 o 



Similar coal to above ......... o 7 



Purplish to light grey clay-shale with plant remains ...50 

 White argillaceous rock, stratified above, massive below, with 



small^quartz-grains included, visible for .... 35 o 



( 78 ) 



