man some. 1 OUTLINE OF GEOLOGY. 37 
Structun of the pre-Telluride formations. — The domal structure 
which has been referred to is clearly seen iu the Animas Valley from 
the Grand Turk Mountain southward. The formations here have a 
Southerly or southwesterly dip, which prevails with some undulations 
to a point below Durango. Westerly dips prevail in general in the 
Telluride quadrangle, and northerly dips are seen in the Uncompahgre 
Valley from Ouray downward. A local structure of interest is seen 
directly cast of Sultan Mountain in the Animas Valley, where the 
Ignacio quartzites, the Ouray limestones, and the lower Hermosa beds 
dip rather abruptly northward. The original extent of this fold is 
not now plain, for these beds are cut off sharply by the great quartz- 
monzonite stocks, beyond which only volcanic rocks are known for 
a number of miles. An interesting evidence of the structure of the 
Paleozoic beds is also seen in [ronton Park. The well-known deposits 
of the Saratoga mine seem quite clearly to be in the horizon of the 
Ouray Limestone, resting upon Algonkian quartzites, as in the Uncom- 
pahgre Valley. They have a westerly dip, which carries them quickly 
below the level of the park, but along the road on the western side a 
very few feet of grits may locally be found beneath the San Juan tuffs. 
These beds of Ironton Park are supposed to represent the known 
eastern limit of the sedimentary section which must be continuous 
beneath the volcanics and which reappears at Telluride, in the drain- 
age of the San .Miguel River. 
Faulting. -Displacements of the volcanic rocks by faulting are 
numerous, but it is in most cases difficult to ascertain the amount 
ami character of the displacement. It is supposed that many of the 
faults seen in the eastern pari of the quadrangle, especially in or near 
the valley of the Lake Fork, have a very considerable throw. In 
other parts of the area dislocation is, as a rule, comparatively slight. 
An exception to this statemenl is afforded by the faults bounding the 
rhyolite body of Porphyry Gulch, near the head of Mineral Creek. 
Here there appears to be a sunken block of Potosi rhyolite bordered 
by faults, some of which are very clearly exposed. Minor faults 
occur particularly in the southern parts of the quadrangle, on either 
side of the Animas. 
In that region there are faults which seem to be of an earlier date 
than the volcanic eruptions, but they can be identified as prevolcanic 
:mly where they are found to stop at the lower surface of the San 
Juan. Two such faults are seen in Cunningham Gulch above the 
Highland Mary mine. Between these faults a block of Ouray lime- 
stone has been dropped, and outcrops of that formation occur on each 
side of the main gulch. These faults do not affect the San Juan tuffs 
ibove the Ouray limestone. 
Landslides. — The Telluride quadrangle is an area of important 
andslides, especially on the steep western face of the San Juan, where 
he Tel iu ride conglomerate rests on soft Cretaceous shales. Two of 
tese slide areas are several square miles in extent and huge blocks 
