The great Tufa Deposit. 147 



The original thickness west of the Donjek must have been at 

 least several feet, and the increase is very marked toward the 

 southwest. The white tufa is washed doAvn from the steep slopes 

 and forms considerable alluvial fans at the mouths of the ravines 

 closely resembling the cones of snow which form in similar posi- 

 tion. After passing the Koidern the narrow valleys were found 

 deeply filled with tufa which had accumulated from the steep 

 mountain slopes. From the divide the upper White River valley 

 was seen stretching forty miles to the westward, and appeared 

 almost completely covered with drifts of snow. On reaching the 

 valley the drifts proved to be tufa, which forms a deep mantle 

 over the country north of the St Elias mountains, and for twenty 

 miles west of the Klutlan forms a desert of drifting snow-white 

 sand into which one sinks from four to twelve inches in walk- 

 ing. A scanty growth of dwarf alder and blueberry bushes has 

 ■ gained a precarious foothold in some places, and a few stunted 

 spruce trees grow in protected spots along the streams. The 

 tufa extends up the mountain sides on the south, covering every 

 surface where the slope is not too steep for it to lie and finally 

 merging with the neve snow, which begins about 1,500 feet above 

 the valley or 6,000 feet above sea level. The valley was covered 

 with a sheet of glacial drift before the deposition of the tufa, and 

 in consequence the drainage is very imperfect. Many small 

 lakes and ponds, usually without outlet, occur scattered over the 

 surface. 



The greatest observed thickness to which the tufa dej^osit at- 

 tains is between 75 and 100 feet. This was seen on the western 

 bank of the Klutlan, where there is no reason to suppose that 

 its original thickness has been increased at the expense of sur- 

 rounding regions except, perhaps, by wind drift. 



Toward the upper end of the valley the thickness of the de- 

 posit decreases very rapidly, and at the entrance to Scolai pass, 

 less than forty miles from its maximum, it appears as a narrow 

 white streak in the freshly cut river banks, exactly as it does 

 along the Lewes and Pell}^, 300 miles to the eastward. The de- 

 posit also appears to decrease in thickness raj)idly toward the 

 north, and there is no indication of any considerable accumu- 

 lation on the gentle slopes of the valley or on the mesas north of 

 White river. 



The gradual increase in thickness of the deposit from east to 

 west is accompanied by an increase in the size of the fragments. 



