44 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
It is probable, however, that nearly all the very rich placers owe their 
origin to secondary concentration. This has been brought about by 
the erosion and dissection of an older placer and the reconcentration 
of the gold contained therein. This process of double sorting is 
probably the chief cause of the bonanzas which are not uncommon in 
the Alaskan placer mines, and will probably also account for those 
irregularities of distribution of the placer gold often within a single 
topographic basin, which are so puzzling to the miner. 
A common form of the enrichment is the dissection of an auriferous 
gravel bench of the slopes of a stream valley by a tributaiy stream. 
This tributary stream carries the gold derived from the bench to the 
main stream, where it is mingled with the gold of the main stream, 
and causes an enrichment of the placers located at and below the 
junction of the two streams. In some instances the gravels of an 
older drainage system, lying often at considerable altitudes above the 
present stream floors, are dissected by the present waterways, and 
the gold contained in the older gravels is thus resorted and recon- 
centrated. Instances of this kind are not uncommon in the Nome 
region, and have been observed by the writer in the Rampart region 
of the Yukon. 
Another form of concentration is that by wave action. In this 
mode of enrichment the waves concentrate the gold which lias been 
deposited in the gravels of the coastal plains. It is in such a manner 
that the marvelously rich beach placers of Nome were formed. 
It seems probable that the study of these questions of reconcentra- ! 
tion will \ct yield important commercial results, even in the better 
known mining districts of Alaska. A practical application of these 
principles would suggest, that the prospector seek to trace old drain- 
age channels and pay special attention to the junction of these with 
the present streams. 
SEWARD PENINSULA. 
During the last season the climatic conditions in the Seward Penin- 
sula were not, by any means, favorable to a large gold output. While 
there were heavy rains in the fall, the months of July and August 
were very dry, and hence but little sluicing was done. It should be 
noted, however, that the experience of the last three years indicates 
that such meteorological conditions are to be expected every third 
year, if not every other year. The output, therefore, is probably not 
nearly as large as it would have been had water been available early 
in the season. Moreover, much of the development was in the nature 
of dead work in p reparation for extensive operations during the pres- 
ent season. Ditches were dug, roads built, and pumping plants estab- 
lished, which will greatly accelerate the prosperity of the district and, 
undoubtedly, will materially increase its gold production. The prob- 
lem of transportation is still a serious one. Under the best conditions 
the landing of heavy machinery and supplies on the Nome beach is a 
