RECENT : CALCAREOUS TUFA. 



327 



water of which contains about 33 parts per 100, 000 all of 

 which indicates that the proportion of carbonic acid dissolved in 

 the water is no more than the normal. 



Again, if the carbonate of lime were thrown down at these 

 places, it is evident that the rush of water would sweep the part- 

 icles into the still reaches, where we would expect to find them 

 settling down and forming beds of calcareous mud, but it is not 

 in the pools that the growth of the travertine takes place. Some 

 other explanation therefore must be found to account for the phen- 

 omenon. 



The dams have all the appearance of being in a state of active 



growth, after the manner of a coral reef, 

 Action of mosses. . . . 



and the resemblance is the more striking 



because the growth is most rapid along the lip of the fall, where 

 the water is most agitated, so that the edge frequently overhangs 

 the pool below (Plate 19), just as the growth of a coral reef proceeds 

 most rapidly where the waves beat upon it. Yet there is no indication 

 in the substance of the travertine of any organic structure, though 

 leaves, twigs, shells, and the remains of insects add materially to 

 its bulk. In one or two instances I have found small portions of 

 it built up of the tubes of some calcareous-secreting larva, but very 

 little is formed in this way. I noticed, however, that wherever 

 the surface of the deposit is in what may be called an active state 

 of growth, it is clothed with the fibres of a minute, bright green 

 moss ; and although this does not appear to have the power of 

 secreting carbonate of lime, since the fibres, so far as I can ascer- 

 tain, do not contain this substance, yet it may afford an explana- 

 tion of the growth of the tufa. The modus operandi I conceive 

 to be as follows : — In the first place, probably when the river 

 is low, the sand and pebbles on its bed become coated with a 

 film of carbonate of lime, filtering out the particles from the water 

 as it flows over them. And here human agency may in some 

 cases, though not in all, come into play ; for the Shans are in 

 the habit of placing fishing weirs on the rapids, built of stakes 

 driven into the bed of the river, and bound together with wattles, 

 leaving an opening at which the fish collect for their passage up 

 stream, and over which the fisherman sits on a small bamboo 

 stage or " machan," waiting his opportunity to scoop them out with 



1 A. Geikie, Text Book of Geology, 4th Edn., Vol. 1.. p 488. 



