32 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
Silverton series. — The next volcanic series after the San Juan was 
termed the "Intermediate series" in the Telluride folio. It was 
very much less developed there than either the San Juan below or 
the Potosi series above. The relations exhibited by the map (PL IV) 
in Potosi Peak and the mountains south of Canyon Creek are typical 
of what was observed throughout the Telluride quadrangle. But, as 
the map clearly shows, this formation increases rapidly in thickness 
eastward and covers, in fact, much the greater part of the Silverton 
quadrangle. It will therefore be known in future as the Silverton 
series, the undesirable term "Intermediate" being discarded. From 
a thickness of 300 or 400 feet in Potosi Peak the Silverton series 
increases to 4,000 or 5,000 feet in observed thickness near the center 
of the quadrangle. This series is a complex of andesitic flows and 
tuffs alternating with rhyolitic flows, flow-breccia, and tuff. In the 
Telluride quadrangle, and in most places in the Silverton, its lowest 
member is a rhyolitic flow or flow-breccia of peculiar character. This 
is well seen in Potosi Peak, along the crest of the range west of 
Mineral Creek, and very near the summit of Sultan Mountain. 
The relation of the different rocks to one another is very variable in 
different places. Rhyolite is seldom typical, and its largest masses 
are reddish flow-breccia — that is to say, lavas of rhyolitic base con- 
taining many inclusions of both rhyolite and andesite. Very seldom 
indeed is the rhyolite free from these inclusions. The andesitic por- 
tion of the series consists of augite- and hornblende-andesites, and the 
relation of massive rock to tuff or agglomerate is extremely irregular. 
In all the central portion of the quadrangle the Silverton series 
appears to be principally composed of andesite, either fragmental or 
in flows. But in the Animas Valley the lowest exposed member of 
the complex, especially along the eastern side from Cunningham 
Gulch upward, is a .rhyolitic flow-breccia of reddish or almost purplish 
color, so full of impurities that its character is not easily recognized. 
It appears either that these rhyolite flows were considerably eroded 
or that they piled up in quite irregular masses at eruption, for the 
surface upon which the succeeding andesitic rocks rest is extremely 
irregular, as will be shown by the final geologic map. The recurrence 
of rhyolitic material at various horizons within the series may be 
observed at many points. The Silverton series consists, then, of an 
irregular alternation of two quite different rocks, erupted doubtless 
from different centers. They are grouped as one series, because the 
complex contrasts as a Avhole so markedly with the products of the 
San Juan epoch below them, which were purely andesitic and entirely 
fragmental as far as seen, and with the Potosi series above, which is 
almost entirely rhyolitic in character. 
Potosi series. — The uppermost series recognized in the Telluride 
quadrangle was termed the Potosi series from its occurrence in maxi- 
mum development in the peak of that name, situated in the extreme 
