26 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE. [buli..18& 
and a magnificent lilac-colored columbine (Aquilegia cmrulea) attain 
the height of their luxuriant beauty. The temperature at this time 
is changeable. The transformation from balmy air, genial sunshine, 
and humming insects to cold winds, dark clouds, and chilling rains 
is by no means uncommon, and ma}^ take place in an hour. The 
month of September is usually delightful. The rain}^ season has 
passed, and toward the end of this month the aspens assume their 
yellow and orange autumnal coloring, enlivening and softening for a 
brief period a landscape in which bleak grandeur is the rather too 
dominant element. 
VEGETATION. 
Most of the slopes, up to a timber line varying from 11,000 to 11,500 
feet, are covered with firs, spruces, and aspens, the last flourishing 
particularly 071 the southern exposures and on the talus slopes. 
Although unsuitable for good sawn lumber, the firs and spruces fur- 
nish round timber of sufficient size and strength for ordinary timber- 
ing in the mines. The basins, being mostly above 11,500 feet, are 
treeless, but are covered in summer with a luxuriant growth of grass 
and flowers. 
TOPOGRAPHY. 
For more complete accounts of the topography of the region the 
reader is referred to the general geological text of the forthcoming Sil- 
verton folio. The present sketch is confined to such considerations 
of topographic form as are necessary to enable the reader to fully 
understand the modes of occurrence of the ore bodies and the condi- 
tions governing their exploitation. 
The lowest point within the quadrangle is found on its northern 
edge in the canyon of the Uncompahgre, where the contours (PI. IV) 
show an elevation of somewhat over 8,100 feet. The highest point is 
Handies Peak, on the extreme eastern edge of the area, with an alti- 
tude of 14,008 feet. Between these extremes the region exhibits such 
abrupt and frequent variations in relief as to constitute a topography 
of the most rugged character. This assemblage of sharp peaks, jagged 
ridges, steep- walled basins, and deep canyons is the result of vigorous 
erosion, acting for the most part on nearly horizontal volcanic rocks. 
The present drainage system is apparently irregular or autogenous — 
that is, uncontrolled to any recognizable degree by regular structure 
in the rocks — and was undoubtedly established in essentially its pres- 
ent form prior to the epoch of glaciation. Of the character of the 
initial land surface, upon which the present streams began their work, 
we know very little. We may assume, with some degree of probability, 
that it was the surface of a broad, dome-like elevation extending 
beyond the bounds of the quadrangle, which, after its original struc- 
tural doming, had been reduced by erosion to an irregular plateau 
near the end of the Cretaceous. This was buried to a depth of several 
