374 The American Geologist. June, 1892 
5. This flour-like gold results chiefly from the oxidation of 
iron pyrites, the gold becoming freed in a very fine condition in 
the brown iron ore which results from the chemical change. 
Briefly speaking the physical features of western Montana are 
similar to many mountainous countries, It is characterized by a 
series of more or less parallel ranges, having a general north- 
westerly and southeasterly direction with intervening valleys. 
The mountains having been formed by upheaval, lateral pressure 
and subsequent denudation. In most of the ranges the central 
core of granite has been exposed with highly tilted slates, lime- 
stones cut by porphyritic and quartzite dikes abutting it. 
At mount Ogden, a mountain of granite (a true granite) which 
is intersected, near a contact with a bed of quartzite, by a series 
of narrow quartz veins holding auriferous iron pyrites and free 
gold—the placer beds are situated along the base of the mount- 
ains, a direct incline at an average slope of 35°, about a mile 
distant from the source. The gold can be found, however, along 
the entire slope intervening. This granite is very much decom- 
posed to a depth of twenty to twenty-five feet, so that in develop- 
ing these veins no blasting was done to that depth, The disinte- 
gration has taken place in the granite owing to change in feldspars 
and the oxidation of iron pyrites which is thickly scattered 
through it, assisted by percolating waters and action of frost. 
The easy removal of this decomposed rock by melting snows, 
assisted by the natural force of gravitation is obvious. 
The manner of the removal of the gold from the veins is, [ 
imagine, that suggested by Wurtz, the formation of persulphate 
of iron in which gold is slightly soluble. This may act as a car- 
rier, a deposition of metallic gold resulting as a nugget; or the 
gold is worn from the rock holding it, and carried by the water 
into the gravels. 
Placer beds vary according to the class to which they may belong 
as regards the nature of the pebbles and boulders, or the presence, 
absence, scarcity or abundance of them, the presence of black 
magnetic sands, the depth of the bed and the arrangement of the 
gravels and boulders in them. 
To give an idea of the nature of a placer bed the following ac- 
count of St. Louis gulch, twenty-five miles east of Helena, as 
illustrating No. 5 is given, At the mouth of the gulch which has 
a general course north and south and from which a creek flows 
