GLACIATION IN THE TELLURIDE QUADRANGLE 507 
rhyolite, making up a maximum of 1,300 feet in thickness. This 
series results in topographic forms similar in some situations to 
those resulting from the erosion of the San Juan series, in others 
more closely resembling the steeper cliffs of the Potosi rhyolite. 
Of the upper, Potosi rhyolitic series, a maximum thickness of about 
1,300 feet remains, consisting of a series of rhyolitic beds, the 
majority of which are flows, but some are tuffs. The area covered 
by this series in the quadrangle is comparatively very small, but 
topographically it is conspicuous, not only because it forms the 
summits of the highest peaks and ridges, but also because it 
weathers in remarkably steep cliff faces, showing often a vertical 
columnar structure, and where the direction or degree of the slope 
changes, bold, sharply angular outlines. 
Below the bedded volcanic rocks in position there occur in order 
the following sedimentary strata, viz.: (1) the Telluride conglomer- 
ate, (2) Cretaceous shales and sandstones, (3) Jurassic shales, 
sandstones, and limestones, (4) Triassic sandstone and _ con- 
glomerate, and (5) some sandstone and conglomerate probably of 
Permian age." 
These sedimentary rocks have influenced in various ways the 
course of events in the geological history of the quadrangle, and 
have been important factors in producing certain conspicuous 
topographic forms found there at present, as for example the steep 
walls of the canyon of the San Miguel River, due in many places to 
the presence of sandstones and conglomerates. ‘For the most part 
these sedimentary strata retain their horizontal position except in 
certain cases in the immediate vicinity of bodies of intrusive 
volcanic rocks; one conspicuous monocline, however, is to be found 
west of the mountain front, and folds somewhat more complex in 
the southeastern corner of the quadrangle. 
LITERATURE CONCERNING GLACIATION IN THIS AND ADJACENT 
QUADRANGLES 
1877. The Hayden Geological Survey—In Appendix A to the Ninth Annual 
Report of the United States Geological and Geographical Survey of the Territories, 
Dr. F. M. Endlich reports, under the heading ‘“‘ Ancient Glaciers in Southern 
t Cross, Engineer Mountain Folio, p. 7, and various other publications there 
referred to. 
