RECENT : CALCAREOUS TUFA. 



329 



the rivers flow for any distance through the limestone. 



In addition to the calcareous dams in the rivers, masses of 



travertine are deposited by every spring and 

 Deposits from springs. , , e . . 



in the bed of every stream, often reaching 



very large dimensions. The scarped cliffs along fault lines 



and the precipitous sides of the gorges are generally festooned 



with huge masses depending like curtains from their crests ; and 



where the conditions are favourable these masses may extend 



completely across the gorge and form a 



'■ Natural Bridges. . „ \ . * _ . n , , , ,, 



Natural Bridge. Of such a nature is the 

 famous ' Natural Bridge ' in the Gokteik gorge, which was for 

 ages used by the Chinese and Shans as a means of crossing 

 this deep canyon with their trains of pack bullocks and mules, 

 and now carries the supports of the great railway viaduct. I have 

 already given an account of this bridge in our Records, 1 and it 

 will be sufficient to say that it appears to have been formed by an 

 accumulation of tufa, deposited mainly by springs on the southern 

 side, which has gradually filled the narrow space between the ver- 

 tical walls of the cleft. Further to the east again, on the descent 

 to the crossing over the Namhsim, before reaching Bawgyo, the 

 railway passes through a cutting several hundred yards long and 

 a hundred feet or more in depth, entirely excavated in travertine 

 deposited by springs issuing from the hill side above the line (Plate 21). 



Where the travertine is deposited by feeble siirings issuing from 

 ,,. fissures beneath the red clav, the latter is 



Clays, etc., impregnat- . 1 



ed with calcareous converted into a very hard, cellular, argill- 

 mattor - aceous limestone, generally of a pink colour, 



and often containing large numbers of recent land shells and leaves. 

 Sometimes also, where the carbonate-laden water of a stream from 

 the limestone area flows through a ravine in the older, non-cal- 

 careous rocks, the tufa will bind together fragments of the latter 

 into a limestone breccia, which may form a very hard, superficial 

 layer several inches thick on the surface of the older rock, and 

 may easily be mistaken for the outcrop of an interstratified band. 

 A freshly broken surface will, however, always show either a min- 

 utely porous or a banded fibrous structure of the matrix, which 

 at once indicates its recent origin. 



1 The Natural Bridge in the Gokteik (Jorge; Records, (Jcol. Surv. 7nrf.,Vol. XXXIII, 

 Pt. 1, p. 49. 



