34 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull.18^. 
of Fullmoon Gulch. A very completely decomposed mass of what is 
probably the same rock is seen on the north side of Mill Creek, and 
still another occurs to the west of Mineral Creek, between Mill Creek 
and the Middle Fork. This rock is characterized by large crystals 
of orthoclase which give it a very marked porphyritic structure. A 
very small occurrence of the same rock is found on the wagon road a 
short distance south of the Yankee Girl mine, and another in the knot) 
north of the National Belle. Intrusions of rhyolitic, and probably 
of other rocks, are known in the surface volcanics of the northeastern 
part of the quadrangle, but their character has not yet been definitely 
ascertained. 
A sheet-like mass of porphyry occurs at the eastern base of Sultan 
Mountain. It has been intruded in quite regular manner in the 
shales just above the Ouray limestone. Several other masses of 
porphyry, more or less distinctly in sheet form, with a few dikes, 
occur at the forks of Mineral Creek west of Silverton. In this 
vicinity all rocks are so extremely decom posed and are so extensively 
covered with glacial and other superficial material that the outline of 
the bodies is very difficult of determination. 
A large body of porphyry of rude laccolithic outline occurs on the 
north side of Canyon Creek on the north border of the quadrangle. 
The representation of this mass on the map (PI. IV) is provisional, 
for the topographic map of this region was revised after the porphyry 
mass had been examined. This mass is an intrusion in the Triassic 
beds and extends for some distance into the Ouray quadrangle. The I 
cliffs of this mass are very notable as seen from the wagon road lead- 
ing up Canyon Creek. The rock is usually very much decomposed. 
Formation* underlying the volcanics. — As explained above, the I 
volcanic rocks rest upon the greatly eroded surface of many older 
formations which have been repeatedly elevated and eroded at this 
general center, producing a complex domal structure. The oldest 
formation displayed is the gneiss and schist series, in which the 
Animas Canyon is eroded for several miles, beginning about one mile 
below Silverton. The rocks of this section have not yet been exam- 
ined in detail, but will be studied in connection with the Needle 
Mountains quadrangle. 
Associated with these gneisses and schists in the Needle Moun- 
tains is a series of quartzites and slates several thousand feet in 
thickness. These rocks cross the Animas a few miles south of the 
Silveiton quadrangle line. A similar section is displayed in the 
picturesque canyon of the Uncompahgre from t e north base of 
Abrams Mountain to Ouray. Here, too, there are some thousands 
of feet exposed, but neither the top nor bottom of the quartzite series 
is shown. On the south the earliest Paleozoic beds or the volcanic 
tuffs cover the quartzites and schists uneonformably. On the north 
the Ouray limestone rests upon their upturned edges. This great 
