328 LA TOUCHE : GEOLOGY OF NORTHERN SHAN STATES. 



a net (Plate 20) ; and I have observed that the leaves and twigs 

 which are caught by the obstruction are often bound together into a 

 solid mass by the tufa deposited upon them, even when the weir has 

 been built but a short time. In most cases, however, I am convinced 

 that the whole of the structure is of natural growth, for they are found 

 in places where no fishing weirs could be built. 



As soon as a film of tufa has been formed, the moss finds in 

 ^ it a favourable soil, and proceeds to cover it 



with a network of fibres. These filter out 

 the particles of carbonate of lime from the flowing water, and the 

 growth of the dam begins. The growing surface, so to speak, is 

 of a spongy texture, and has a mammillated appearance caused by 

 aggregations of minute granules of carbonate, of lime mingled with 

 vegetable fibres. No doubt the decomposition of the latter, 

 as they become buried in the growing tufa, assists by the formation 

 of carbonic acid to bind the mass together. The more rapid 

 growth along the lip of the fall may perhaps be brought about 

 because it is here that the mosses are most exposed to the light 

 which is favourable to their growth. Thus, although the accumu- 

 lation of the travertine is mainly due to purely mechanical causes, 

 it depends to some extent on the growth of a living organism, and 

 it is this factor that givts to these dams their outward resemblance 

 to coral reefs. 



I have already mentioned that, as regards the Nam-Tu, the 



calcareous dams are only to be found in the 

 Occurrence in Nani-Tu. , , . , , , , 



upper part 01 the river, before it enters the 



deep gorge above Ta-pangtawng. The reason seems to be that, 

 as it passes through this gorge, it receives so great an influx of 

 pure water from the sandy and argillaceous hills on its right bank- 

 that the particles suspended in it become too greatly diffused to 

 enable the deposition of a film of tufa to take place. There is a 

 very fine series of dams at the point where the Namma joins the 

 Nam-Tu, at the railway bridge above Hsipaw (Plate 17), but they end ab- 

 ruptly at the conflux, and below this none are to be seen, at least 

 so far as I have been able to follow the river, that is to say, 

 down to the ferry at Tong-ang. In the rivers that flow to the 

 Salween from the Loi Ling area and Loi Twang they seem to 

 be rare, at any rate within the part of the country now described, 

 but there are some good examples in the Nam-hen near Kehsi- 

 Mansam, and they are almost certainly to be met with wherever 



