326 LA TOUCHE : GEOLOGY OF NORTHERN SHAN STATES. 



or reduced to a mere trickle of water in the dry season, and in 

 the natural course of events the carbonate is thrown down as a 

 precipitate as the water evaporates ; but the case of the larger 

 perennial rivers is different. In these the deposits take the form 

 of dams or natural weirs, from a few inches to six feet or more 

 in height, extending across the river from bank to bank, and as 

 level along the crest as if they had been bailt by human hands. 

 A remarkably fine example is seen in the foreground of the view of 

 the Mansang falls (Frontispiece). They are especially numerous in the 

 Namma and Namyau, above their conflux with the Nam-Tu; in the 

 upper part of the Nam-Tu itself ; and in most of the tributaries of these 

 rivers (Plates 17, 18, and 19) ; but there are none in the Nam-panhse 

 above the Gokteik gorge or in the Namhsim ; because these rivers flow 

 through non-calcareous rocks. 



The dams are to be found at those points where, in an ordinary 

 river, we would, expect to find rapids separat- 



Causcs of growth. . ■< r ,i , .. . . , . 



mg one reach from another ; and it is this 

 peculiarity in their position that constitutes the chief difficulty 

 in accounting for their formation. The first explanation that oc- 

 curred to me was that in such places the water is thrown into 

 violent agitation, thus affording a ready means of escape for the 

 carbonic acid holding the carbonate of lime in solution, and that 

 the latter was thrown down on the spot. But it is evident that 

 most of the carbonic acid must escape very soon after the water 

 reaches the open air, and that the first rapid encountered should 

 insure its dissipation. That this is actually the case is shown by 

 the colour of the water seen when at rest in the deep pools ; for 

 it has a milky, greenish-blue tinge, exactly like that of a reservoir 

 which has been treated with quick-lime in order to soften the water, 

 caused by the minute particles held in suspension. Moreover, the 

 water of these rivers has been found by analysis to contain no 

 more than the usual proportion of solid matter in solut n. A sample 

 from the Namyau at Se-In, (G 2) collected 

 ^ Solid matter in solu- near the end of ]yr arch) when he r j ver was 



almost at its lowest point, gave only 22*3 

 parts of solid matter per 100,000 ; while that of the Nam-Tu at 

 Hsipaw, where there are no calcareous deposits, showed even a 

 higher proportion, 29 parts per 100,000, at the same season. 

 Both of these ratios are near the average amounts contained in 

 the waters of ordinary rivers, less indeed than in the Thames, the 



