384 H. CoUeU^Features in the Geological Structure of the [No. 4, 



XYIL—On certain Features in the Geological Structure of the Myelat 

 District of the Southern Shan States in Upper Burmah as affecting 

 the Drainage of the Country. ^By Brigadiee-General H. Collett, 

 0. B. Communicated by Dr. D. D. Cunningham. 



[Received August 18th ;— Read Nov. 17th, 1888.] 



There are some curious features in the geological structure as 

 affecting the systems of drainage of the Myelat district of the Southern 

 Shan States in Upper Burmah which appear to me to be novel and 

 worthy of attention. 



The general geological formation in this part of the great Shan 

 plateau may be described as water- worn limestone with occasional inter- 

 posed sheets and boulders of conglomerate underlying a sedimentary 

 deposit of finely divided red clay, which varies in thickness from a 

 thin superficial covering up to three or four hundred feet according to 

 the amount of denudation it has undergone. This mantle of red clay 

 at one time certainly overspread the whole country, probably at a nearly 

 uniform level, for patches of it, like raised beaches, are seen clinging to 

 sheltered hollows in the black limestone ridges which rise through it 

 in long parallel folds, remnants which no doubt mark the ancient level 

 of the red clay as deposited in the quiet depths of an ocean or large 

 lake. The underlying limestone wherever exposed to view is seen to 

 have been worn into rounded hollows and projecting bosses apparently 

 by the action of water at a time when it was exposed to sub-aerial 

 denudation, and it is, like limestone in other parts of the world, full of 

 clefts, crevices, and caverns which communicate with each other to 

 from subterranean channels into which a great part of the superficial 

 drainage of the country disappears. 



To such an extent is this the case in the Myelat that, though the 

 district is traversed by distinct ranges of hills, the valleys lying between 

 them have not as a rule been excavated by water- courses in the ordinary 

 sense of the word, nor do they drain into rivers, but into holes in the 

 ground, and thus we have the strange spectacle repeatedly presented to 

 us of large basin-like depressions, and even narrow valleys, ending in a 

 cul de sac and possessing no apparent outlet for the discharge of their 

 drainage. 



These depressions (which vary in size from a punch-bowl of a 

 few feet in diameter to areas covering, as I roughly estimate, three 

 or four square miles, and occasionally not less than two hundred feet 

 in depth) are of an altogether different character from the " swal- 

 low holes " or " sinks " common in districts of calcareous rocks in 



