PLACER DEPOSITS. 517 



in which they may be made available by man requires not only the gold- 

 bearing- rock, which her agencies may grind up into sand and gravel, but 

 the sifting power of rapid streams, which may carry down the lighter and 

 coarser material, and a suitable channel, in which the heavier particles may 

 lodge, as in the riffles of a sluice-box. All mbuntain gravels, all sands of 

 rivers coming from the mountains, contain a certain amount of gold, but it 

 is only under peculiarly favorable conditions that the gold is so concentrated 

 as to render the gravel or sand remunerative to the labor of man. Among 

 the most favorable of these conditions is a comparatively narrow channel, 

 having a hard and compact bed-rock and ridges or bends in its coui-se, 

 which, by causing a partial arrest in the rapidity of the current, shall allow 

 the heavier particles of gold to settle to the bottom and hold them there 

 when once they have settled. 



From this point of view there is a very evident reason why California 

 gulch should have furnished rich placers and why the gold which may exist 

 in Iowa and Evans gulches should not yet have been extracted, even though 

 the detrital material which has been carried down either gulch should orig- 

 inally have been equally rich in gold. California gulch, as has already been 

 explained, is a valley of erosion, formed entirely by the action of running 

 water and since the Glacial period. It has, therefore, a bottom or bed 

 of hard rock; its transverse section is V-shaped and therefore, like the 

 Spitzkasten, favorable for the concentration of heavy particles at its bot- 

 tom. When comparatively full of water, its numerous bends formed eddies 

 in the down-flowing currents and allowed a longer time at these points for the 

 settling of the suspended particles; and, as it cuts across many different forma- 

 tions in its course, its bed must have transverse ridges, which have caught 

 some of the gold and prevented it from being carried farther down the stream. 

 p]vans and Iowa gulches, on the other hand are glacier-carved valleys; their 

 courses are straight, their bottoms broad and comparatively smooth. The 

 moraine material with which they are largely filled has not been subjected 

 to the sifting or jigging process, to which gravel is subjected in the bed of a 

 stream. Moreover, as shown in Sections M, N, and O, the lower part of their 

 present bed is cut, not out of rock, but out of the loose gravelly formation of 

 the Lake beds. It is not probable that this later bed, along which the mate- 



