160 ALASKA AND ITS MINERAL RESOURCES 



treating cordilleran glacier. These ancient silts and the benches 

 or terraces that fringe the mountains all over the interior of 

 Alaska np to 3,000 feet above the present sea-level point to a 

 comparatively recent submergence of the country to this amount. 

 The American geologists are inclined, however, to attribute a 

 lacustrine origin to part at least of these silts. The absence of 

 marine fossils in them is admitted by Dr Dawson to be negative 

 evidence against their marine origin. From an economic point 

 of view, these silts are of little importance, however, as the gold 

 contained in them would be so finely divided that it probably 

 could not be extracted at a profit. 



It is otherwise, however, with the terrace gravels, which are also 

 very widespread throughout the interior. When these occur at 

 moderate heights above the present streams and evidently rep- 

 resent earlier stages in the cutting down of their valleys, they 

 may naturally be expected and indeed are often found to con- 

 tain considerable gold, which it may pay to extract. In the 

 Cassiar mining district quite a large proportion of the gold was 

 derived from terrace gravels. The higher terraces, which are 

 not confined to present valleys, but cross divides and sometimes 

 form plateaus, must have been worn down or redistributed by 

 broader bodies of water, which would be less likely to concen- 

 trate the gold than river waters. They have already been ob- 

 served at 1,500 feet elevation, and if the hypothesis 'of submer- 

 gence expressed above is correct, should be found up to 3,000 

 feet ; they are probably of little economic importance. 



Ancient river gravels that have been protected from erosion 

 by a covering of recent lava have not yet been noted in the 

 Yukon valley, though recent flows of basaltic lava occur at vari- 

 ous points from the lake region of the Lewes river down to St" 

 Michael island, 60 miles north of the mouth of the Yukon. In 

 the Upper Stikine valley such an old river channel, in which 

 auriferous gravels had been protected by a recent flow of basalt, 

 is cut through by the modern stream and has caused a notable 

 enrichment of its bars immediately below. It is a question, how- 

 ever, whether modern erosion in the Yukon valley is sufficiently 

 deep and active to expose such channels if they do exist there. 



Another source of gold, which occupies an intermediate posi- 

 tion between original and detrital deposits, is what is generally 

 known as fossil placers or conglomerate beds, within a geological 

 rock formation which is made up of material resulting from the 

 wearing down, generally on an old shore line, of older gold-bear- 



