NUMMULITIC SERIES. 15 



the presumption is in favor of a great thickening of the series to the south. 

 The intimate association of the sandstone with the limestone in the 

 section at Theria Ghat^ and the wholesale substitution of one for the 

 other between Cherra and Lairung-ao^ would lead one to anticipate much 

 variation in the composition of contiguous sections. 



There are equally important modifications to be noted in the num- 

 muhtic strata along the strike of the formation. For some distance 

 to east and west on the plateau the sections are very similar to that on 

 the Cherra ridge. But all the details of the section are much altered 

 in the North Cachar district where I crossed the plateau in 1865. The 

 cretaceous beds show at the northern edge at a low level resting on the 

 gneiss; and I crossed from the cretaceous to the yoimger Tertiary 

 rocks (which there form the highest ground) without detecting any charac- 

 teristic nummulitic beds. I was marching by long stages,, and the 

 rocks are greatly concealed, but if the limestone were present even to the 

 same extent as on the plateau at Cherra, I could scarcely have failed to 

 notice it. It may be that there too the formation thins out to the norths 

 and that the present outcrop is on this attenuated .portion. 



Eegarding the western region our information is much less vague. 

 In the Garo district the high plateau is almost entirely formed of crys- 

 talline rocks, I believe of the gneiss. All the upper part of the 

 scarp is of this rock, against the base of which the unaltered sedimen- 

 tary beds turn up or abut. We have no knowledge as yet of any 

 outliers of the younger strata that may exist here on the higher ground. 

 At Tura, the only high point I was able to ascend to, there is no vestige 

 of a capping rock. I can only speak of what is seen in the low 

 flanking hills where the whole sedimentary series is weU exposed. On 

 the Sumesari river, sixty miles to the west of Theria, the nummulitic 

 limestone rests, as elsewhere, directly and conformably upon the cretace- 

 ous sandstone. There is not more than about forty feet of it ; and it 

 is the only band that occurs. But it is not in respect of thickness that 



( 165 ) 



