150 C. W. Hayes — Expedition througli the Yukon District. 



green to a milky white." This turbidity has been attributed to 

 the glacial source of the river, but glaciers could scarcely supply 

 such an enormous quantity of mud unless acting under peculiar 

 conditions. The presence of this great deposit of unconsolidated 

 material, which is being ground up by the ice and removed by the 

 englacial streams, affords a ready explanation of the turbidity 

 of the water. The highly vesicular character of the tufa permits 

 a much larger amount of it to be held in suspension than of sedi- 

 ment derived from compact rocks. 



Tertiary volcanic Activity. 



Evidence of volcanic activity, geologically recent though very 

 much more remote than the eruption of the tufa deposit, is 

 somewhat abundant. Perhaps the most striking example of 

 such activity is seen in the basaltic mesa at the junction of Pelly 

 and Lewes rivers. This lava flow took place after the river 

 valleys had been eroded j^erhaps below their present levels and 

 extended entirely across the valley. The river has since cut 

 through the barrier, leaving only a few fragments of the basalt 

 resting on the granite on the western side of the channel. This 

 lava flow probabl}^ came from two or- more vents ; one about ten 

 miles north of Selkirk still retains the form of a symmetrical 

 cone, and according to the native accounts has a small lake upon 

 its summit, probably occupying the crater. A second vent was 

 the high hill on the western side of the Yukon, about four miles 

 northwest of Selkirk. Between the Yukon and St Elias moun- 

 tains black vesicular lava was seen at a number of localities, and 

 north of the upper part of White river are broad mesas which 

 appear to be formed of black lava. These are all probably of 

 Tertiary age. 



Glacial Phenomena. 



Existing Glaciers. 



So far as known the existing glaciers of Alaska are confined to 

 a narrow belt along the southwestern coast. Although the high- 

 est land lies in the coast belt, this is not the sole or chief reason 

 for the notable absence of glaciers in the interior, excejjt in so 

 far as climatic conditions are thereby modified. There are 

 numerous points in the Yukon basin from which practically all 



