20 LATOUCHE: GEOLOGY OF NORTHERN SHAN STATES. 



therefore, as a denuding agent than either the Irrawaddy or the Mek- 

 hong, for these rivers lose their power of eroding the rocks where 

 they issue into the plains, that is, at a much greater distance higher 

 up their course, relatively speaking, than in the case of the 

 Salween. To this cause may be attributed the great depth of its 

 valley as compared with those of the Mekhong and the tributaries 

 of the Irrawaddy, where these run parallel to it in Yunnan. Captain 

 Gill says that on the road between Bhamo and Tali, where he 

 crossed it in 1877, the bed of the Salween was 1,300 feet lower than 

 that of the Mekhong, and nearly 2,000 feet below that of the 

 Shweli. These considerations lead to the conjecture that the 

 Salween is of far greater age than either of the other rivers, and 

 that the narrowness of its valley is due to the encroachment of 

 these latter upon its original drainage area. It is perhaps only 

 the great depth of its valley which has saved it from being diverted 

 into one or other of the channels on either side of it. 



The curious circumstance, that the tributaries of the Salween 

 in the Shan States usually enter the river by a cascade or cataract, 

 mentioned by Sir J. G-. Scott (Gazetteer of Upper Burma, Part II, 

 Vol. Ill, p. 93), may be partly due to a more rapid erosion of its 

 bed than the side streams can keep pace with, but is more prob- 

 ably caused, as Sir George Scott himself suggests, by the enormous 

 rise of the river during the rains, which is on an average between 60 

 and 70 feet. This rise would pond back the waters of the side streams 

 and cause them to drop their burdens of gravel and boulders at their 

 mouths. 



The effect that the geological structure and composition of the 

 Effect of rock forma- r0cks has in modifying the forms assumed 

 tion on character of by the stream valleys may be well studied 

 streams - in the Shan States. On the plateau proper, 



where limestone is the prevailing rock, denudation proceeds more or 

 less evenly over the whole great deal of it beng subter- 



ranean, due to solution of the limestone. The insoluble matter 

 contained in the rock remains on the surface, and forms a thick 

 mantle of red clay, through which the solid rock only appears at 

 intervals. The smaller valleys are broad and shallow, with gently 

 sloping sides, and are usually traversed by sluggish streams, often 

 bordered by morasses. It is only where several streams have united 

 and have sufficient power to cut through the covering of clay, that 

 the rocks beneath are exposed, and then, if the conditions are other- 

 wise; favourable, a narrow gorge, bordered by precipitous cliffs of 



