36 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
has been found in the upper part of the formation. These strata are 
seen in Canyon Creek and in the southwestern corner of the Silver- 
ton quadrangle. All the Paleozoic and Mesozoic formations named 
exhibit a domal structure by their prevailing dips to the north or to 
the south, away from the center in the Silverton quadrangle. Beyond 
the borders of this quadrangle the other formations of the Mesozoie 
appear. 
Prevoicanic surface and the Telluricle conglomerate. — The Hayden 
map represents the volcanic complex of the San Juan as resting on 
a general plane surface eroded across many different formations. 
Over considerable stretches the base of the volcanics is represented 
as approximately level. A detailed examination of the Telluride 
quadrangle showed that the San Juan tuff was in that region every- 
where underlain by a conglomerate free from volcanic material but 
containing pebbles of very hard sedimentary rocks, Mesozoic and 
Paleozoic, with schists, granites, and Algonkian quartzites, such as 
may be seen in the Needle Mountains; that is to say, this conglomer- 
ate contained in its pebbles the record of an enormous erosion, such 
as would be required to produce the surface upon which the volcanic 
formations in general rest. 
This conglomerate below the volcanics was first called the San 
Miguel conglomerate. It is now necessary to replace that name by 
another, as it has been found that there is a prior use of that term for 
a Cretaceous formation in Texas. It is therefore proposed to rename 
this conglomerate below the San Juan tuff the Telluride conglomer- 
ate, or formation, on account of its typical exposures and relations, 
clearly seen about the town of that name and, indeed, throughout 
the Telluride quadrangle. The Telluride formation varies greatly 
in thickness; it is almost 1,000 feet thick in Mount Wilson, on the 
western border of the Telluride quadrangle. It is 700 feet thick in 
Sheep Mountain, southwest of Trout Lake, and from that point thins 
gradually eastward to Sultan Mountain in the Silverton quadrangle, 
where, on the eastern slope, it has an average thickness of about 30 
feet. To the east of the Animas this conglomerate is not continuously 
developed, but is found occupying hollows in the surface of gneisses 
and schists. The San Juan tuffs resting upon it bury the small promi- 
nences separating these hollows, and the Telluride conglomerate is, 
as a rule, not conspicuous. It is ver}' distinctly seen, howeA^er* 
beneath the San Juan on both sides of Whitehead Creek. The same 
thinning of the Telluride conglomerate occurs between Telluride and 
the exposures in Canyon Creek. The conglomerate is very clearty 
exposed upon the wagon road leading up the north side of Canyon 
Creek, and is probably continuous along its southern side to the valley 
of Red Creek, but beyond that to the east is seen only in discon- 
nected exposures. It is thought probable that the same format ion 
occurs beneath the volcanics in the lower country east of the Uneoin- 
pahgre River in the Ouray quadrangle. 
