3 i2 POPULAR SCIENCE MONTHLY. 



knowledge does not permit me to state if the subfossil species of 

 the banks have their living representatives, either specific or generic, 

 in the almost wholly noncalcareous waters of the existing river. 

 The question from more points than one is interesting, and deserves 

 more than passing attention. It may be remarked in this place 

 that the only other fluviatile invertebrate which I found in these 

 waters was a white siliceous coating sponge, whose statoblasts were 

 well visible to the naked eye. Unfortunately, the loss of my speci- 

 mens has prevented determination, a circumstance the more to be 

 deplored as these fresh-water sponges are the most northern in habit 

 known to the zoologist.* 



There is evidence of another kind pointing to a comparative 

 newness of much of the present course of the Yukon. The feature 

 has been noticed alike by nongeographers and geographers, and by 

 geologists as well, that the arm which carries the greatest volume 

 of water does not everywhere occupy the main orographic valley. 

 Thus, as Dawson has well pointed out, in coming up the stream the 

 valley of the Big Salmon appears to be more nearly the continuation 

 of the main valley below than that which still (and properly) con- 

 tinues to be designated the Lewes (Yukon) above; and this is still 

 more markedly the case with the Hootalinqua (Teslin-too or New- 

 berry River) at the confluence with the Thirty Mile. Even the 

 valley of the Pelly at its junction with the Yukon, near Fort Sel- 

 kirk, would perhaps to most persons suggest itself as the main chan- 

 nel of erosion. There is no hardship to geological facts in invoking 

 the aid of great displacements to account for a condition which to 

 my mind is well impressed upon the landscape; -for, even without 

 the proper or fully satisfactory evidence in hand to support the 

 view, I fully believe that the greater part of the upper Yukon tract 

 only recently emerged from a lacustrine condition. Nor is it to 

 me by any means certain that this emergence or final reconstruction 

 of the land surface into valley tracts need be more than a few hun- 

 dred years old, or necessarily older than the deposition of the vol- 

 canic ash, which is hypothetically carried back to Dawson to a pos- 

 sible five hundred years or so. If it should be objected that we 

 know of no such rapid change in the configuration of a land surface 

 brought about by aqueous agencies, it might be answered that the 

 mechanics of erosion in a pre-eminently drift-covered region, under 

 subarctic conditions and with the influence of a most powerful 

 and energetic stream near by, have neither been studied nor 

 observed. 



* Professor Russell, in discussing the flood-plain deposits of the Yukon about the mouth 

 of the Porcupine River, says that "fresh-witer shells were frequently observed in the finer 

 deposits." Unfortunately, no statement is made of the types which they represent. 



