SILURIAN SYSTEM : LOWER NAMHSIM STAGE. 



135 



Namhsim, passing to the west of Kunkaw, and then northwards 

 through Hunang and Kunhawt to and beyond Tawngma (F 1). The 

 boundary with the older rocks is deeply eroded and exceedingly 

 tortuous, on account of the enormously deep, narrow ravines by 

 which the whole of the State of Tawngpeng is intersected. Out- 

 liers of the sandstones, occupying the higher points of the hills, 

 are numerous. The boundary is sometimes 



Conglomerates and mar ked by beds of coarse conglomerate, con- 

 grits at base. .. . ... -iiii t 



sisting of water-worn pebbles and boulders ol 



the Chaung-Magyi quartzites. Where these are absent the lower 



beds of the sandstone series are usually coarse-grained, and for 



some distance up from the base they are strongly felspathic, and 



generally of purplish or bluish grey colours, like the Orthonota beds 



on the Memauk spur described above. These gradually become 



less felspathic, passing into fine-grained, brown sandstones, with 



layers of a hard, white, very fine-grained quartzose sandstone. 



Fossils are somewhat rare, but occasionally the cast of an Orthonota 



or a fragment of a trilobite may be found. 



The inner boundary of the sandstone series is always easily 

 j rb ^ discernible, on account of the contrast with 



the overlying Plateau Limestone, beneath which 

 the sandstones usually form well marked and precipitous scarps. 

 It is to be noted, however, that where the existing drainage has cut 

 down to the base of the sandstones, and has exposed their plane of con- 

 tact with the underlying Naungkangyi group, the basement beds are 

 very different from those just described, along the outer boundary. 

 There are no conglomerates, or even traces of a coarsening of the 

 sandstones as the base is approached, and wherever a fairly clear 

 section is obtainable, as on the Panghsa-pye saddle, on both sides 

 of the Nam-non ravine, and in the gorge of the Nam-Tu, the fine- 

 grained sandstones are seen to rest with apparent conformity upon 

 the graptolite beds beneath them. The absence of the latter, how- 

 ever, in some places where they ought to occur, as in the deep gorge 

 of the Namhsim at Hkyawngtawng, seems to show that there may 

 have been some erosion of the underlying beds before the sand- 

 stones were deposited, and that consequently there may be a slight 

 amount of unconformity. In any case it is evident that the sand- 

 stones were deposited in a sea shallowing rapidly towards the north, 

 and that at this time the older rocks of the Chaung-Magyi series formed 

 a land area in that direction. 



