190 THE UPPER YUKON 



to occur near the surface of terraces about 200 

 feet in height, as well as on lower terraces and 

 river flats down to within about ten feet of the 

 actual river level in August and September. 

 It was also detected in some places on the slop- 

 ing fronts of terraces. 



"The thickness of the layer was no doubt 

 originally pretty uniform, and it still retains 

 this uniformity where it rests upon wide flat 

 terraces. Its average normal thickness for 

 the Pelly, as a whole, was estimated at about 

 five inches, but this is somewhat exceeded 

 along the part of the river immediately above 

 the MacMillan River. On the Lewis (the 

 Yukon) below Rink Rapid its normal thick- 

 ness is about a foot, but above this point it be- 

 comes much less, and where last seen, at Cari- 

 bou Crossing, is not over a half-inch thick and 

 only to be recognized when carefully looked 

 for." 



Dr. Dawson is of the opinion that this vol- 

 canic eruption probably came from "Mount 

 Wrangell, as it is the nearest known volcano," 

 and that the extent of the eruption covered a 

 radius of possibly 25,000 miles. As to its 

 probable date, he says : "The rivers have not 

 certainly cut their beds perceptibly deeper 

 since the deposit occurred on their flood flats, 



