RECENT : CALCAREOUS TUFA. 



325 



Naungkangyi beds . . . Variegated. Mainly yellow, sometimes 



orange red, reddish brown, or lilac. 



Hwe Mawng beds . . . Lavender or lilac when dry becoming 



purple when moist. 



Namhsim (lower) .... Brown and generally sandy. 



Namhsim (upper) .... White or buff. 



Plateau Limestone . . . Bright red. On the Permo-Carbonif- 



erous rocks blue-grey, or yellow. 

 Namyau beds .... Violet red, or dark purple. 



(b) Calcareous Tufa. 



The enormous extent to which the limestone of the plateau 

 is being removed in solution by percolating 



Calcareous dams. T , , . , , . . - . . 



waters has already been alluded to, and it is 

 not surprising to find that, when the water comes again to the 

 surface in springs and rivers, and is either evaporated or loses the 

 carbonic acid which keeps the carbonate of lime in solution, the 

 deposits thrown down should reach correspondingly huge dimensions. 

 Indeed I doubt whether any other limestone tract can show deposits 

 of this kind of such magnitude, at least in the open air. In the 

 ordinary ' Karst ' region the evaporation usually takes place as 

 the water trickles into the caverns and hollows worn out cf the 

 rock, with the formation of stalactites and stalagmite ; but in the 

 Shan States there are no open caverns in the great bulk of the 

 limestone, owing to its universally shattered condition, which causes 

 the mass to settle down as underground solution proceeds ; though 

 in the super-jacent, more compact, Permo-Carboniferous limestones 

 caverns are common enough. Thus the carbonate of lime which 

 would ordinarily be deposited on the walls of caverns and fissures 

 is in this region brought to the surface and thrown down in the 

 open. The brccciated structure of the rock also allows water to 

 percolate freely through the mass in all directions, and this no 

 doubt adds to the rapidity with which it is dissolved. 



The mode of deposition of the carbonate of lime as travertine 

 in the beds of the rivers is of some interest. 



Mode of occurrence. , , ' 



and presents some problems not altogether 

 easy to solve. So far as the smaller watercourses are concerned 

 there is no difficulty, because these are either entire ly drier] up 



