28 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
raer. But in many instances it is plainly the result of one or more 
rock slides of some magnitude, by which large masses of the cliff have 
fallen and slid out onto the floor of the basin. 
The lakelets lying in these basins or amphitheaters are usually rock- 
rimmed and often of considerable depth. The existence of the lakes 
depends merely upon the characteristic form of the main basins or 
cirques in which they lie. This form is immediately due to the erod- 
ing action of local glaciers which once occupied the basins. The out- 
let of Silver Lake flows over a sheet of hard, massive andesite, whose 
surface has been rounded and scored by the ice. The lake basin itself 
lias been excavated, however, in softer, more readily erodible volcanic 
breccias. Ordinary erosion by running water would have been power- 
less to excavate such a basin below the level of its present outlet. 
Ice is the only eroding agent that could produce such a result. 
The evolution of the topography has undoubtedly been influenced 
to some extent by the fissures and veins that are so abundant in this 
region. But it is difficult or impossible to reduce their effect to the 
form of a simple and general statement. Mineralized fissures are 
frequently lines of comparatively easy oxidation, rapid decomposi- 
tion, and ready erosion. Thus very many of the lodes determine the 
position of the cols or saddles in the ridges, and can be distinguished 
from a distance by the yellowish color of their surface detritus. Yet 
this is by no means universally true. It often happens that in the 
present stage of erosion the croppings of a vein traverse an even 
slope or cross the summit of a high peak. As an example of the last 
ma} 7 be cited the important Titus ville vein where it crosses Kendall 
Mountain. It is plain that in some cases the fact that erosion has 
resulted in cliffs rather than steep slopes is due to the presence of 
nearly vertical fissures. A system of such fissures running nearly 
northeast and southwest is responsible for the cliffs which separate 
Silver Lake Basin from Arrastra Gulch. 
In this elevated region the disintegrating action of frost and the 
active erosion far outstrip the chemical processes of decay which in 
mild and moist climates are efficient in reducing the rocks to soil. As 
a consequence the high peaks and ridges are usually composed of bare 
rock, while their lower slopes are largely made up of talus or ' ; slide 
rock," as it is locally called. This talus has concealed in a most 
effectual manner much of the rock once scoured clean by the ice, and 
has restricted most of the prospecting to the high peaks and ridges 
where the veins are exposed. 
In the vicinity of Red Mountain and Ironton much of the topog- 
raphy has a peculiar hummocky character, which is the result of 
landslide action. This feature is particularly striking on the northern 
and western flanks of Red Mountain. Several of the Red Mountain 
mines, such as the Yankee Girl, Genesee- Vanderbilt, Guston, Silver 
Bell, Paymaster, and others, are in this landslide area. But the ore 
