176 TIBET. Chap. XXIV. 



owing to the rarity of the atmosphere and the low tension 

 of its vapours), the drought is accompanied by perfect 

 transparency, and the atmosphere is too attenuated to 

 support the dust raised by the wind. 



We descended in the afternoon, and on our way up the 

 Lachen valley examined a narrow gulley in a lofty red 

 spur from Kinchinjhow, where black shales were in situ, 

 striking north-east, and dipping north-west 45°. These 

 shales were interposed between beds of yellow quartz 

 conglomerate, upon the latter of which rested a talus of 

 earthy rocks, angular fragments of which were strewed 

 about opposite this spur, but were not seen elsewhere. 



It became dark before we reached the Cholamoo lake, 

 where we lost our way amongst glaciers, moraines, and 

 marshes. We expected to have seen the lights of the camp, 

 but were disappointed, and as it was freezing hard, we 

 began to be anxious, and shouted till the echos of our 

 voices against the opposite bank were heard by Tchebu 

 Lama, who met us in great alarm for our safety. Our 

 camp was pitched some way from the shore, on a broad 

 plain, 16,900 feet above the sea.* A cold wind descended 

 from Donkia; yet, though more elevated than Yeumtso, the 

 climate of Cholamoo, from being damper and misty, was 

 milder. The minimum thermometer fell to 14.° 



Before starting for Donkia pass on the following morning, 

 we visited some black rocks which rose from the flat to the 

 east of the lake. They proved to be of fossiliferous lime- 

 stone, the strata of which were much disturbed : the strike 



* This, which is about the level of the lake, gives the Lachen river a fall of 

 about 1500 feet between its source and Kongra Lama, or sixty feet per mile 

 following its windings. From Kongra Lama to Tallum it is 140 feet per mile ; 

 from Tallum to Singtam 160 feet; and from Siugtam to the plains of India 50 

 feet per mile. The total fall from Cholamoo lake to its exit on the plains of India 

 is eighty-five feet per mile. Its length, following its windings, is 195 miles, 

 upwards of double the direct distance. 



