36 GRIESBACH : GEOLOGY OF THE CENTRAL HIMALAYAS. 



glacial action. Both these small lakes are completely rock-surrounded, 

 with an insignificant outlet, differing in every characteristic from 

 known glacial basins. 



The hill ranges immediately south-west of the Naini Tal lake 

 suggest the true origin of the Kumaun lakes generally. The lime- 

 stone which rests conformably on calcareous shales in that area, is, like 

 mostlimestones, much jointed, and in consequence all the drainage finds 

 its way through the joints into the underlying shales. These become 

 disintegrated and are gradually carried away, whilst the thick limestone 

 beds sink down to the level which was formerly occupied by the shales. 

 This phenomenon may be seen in the range which forms the south- 

 western side of the lake, and at many other localities in these hills. 

 Under certain circumstances such a process must result in the forma- 

 tion of " cirques/'- — circular valleys — Einbruchsthaler — which become 

 filled with water as soon as a sufficiently water-impervious layer of 

 alluvium has formed in the basin resulting from this underground 

 denudation. 



Another factor which possibly helped this denudation still further, 

 is the folding and wrinkling process which must be supposed to be 

 going on still. It is conceivable that this action has resulted in 

 the elevation of the outer edge of the Kumaun hills, north of which 

 are now the lakes, dammed up by this rising outer edge. 



Even with both these factors in active operation, the possibility 

 is not excluded of some of the f * dams " having been supplemented by 

 rock slips, coming down from the hill-slopes which inclose the 

 valleys. 1 



Several lakes of large extent exist in the Hundes province of Tibet. 



The Manasarawar lakes (Tso Mapang) of Hundes 



with the Rakas T31 (Tso Long) lie north of the 



easternmost corner of the area which I now report on. They belong 



to the drainage basin of the Sutlej, which, during part of the year, flows 



from the Rakas Tal. Both lakes are connected by a running channel. 



1 Whilst correcting the proof-sheets of this " Memoir," Pt. 4 of Vol. XXIII of the 

 " Records " reaches me ; in it Mr. Middlemiss suggests a similar origin for the Naini 

 Tal lake. 



( 36 ) 



