6 S. W. WILLISTON AND E. C. CASE 
in all directions, but chiefly east and west. The basin thus formed 
is about two and a half miles in its greatest extent, in a north-and- 
south direction. Its very steep walls, for the most part about seven 
hundred feet in altitude, attain their greatest height in the north- 
west part, where the altitude may exceed eight hundred feet, and 
where the Permian exposures are the greatest. 
The erosion of the floor of this basin, acting on the beds of alter- 
nating sandstones and clays, has formed a series of steps or low 
cliffs, which for the most part dip at a small angle toward the west. 
Toward the sides and upper end of the cafion these ridges become 
more prominent, frequently forming high bluffs and cliffs. The 
lowermost beds in the canon are deep chocolate-colored sandstones 
and fine conglomerates; the latter weather into low, rounded hills, 
frequently streaked with greenish layers. Bone fragments were 
found in these layers in various places in the basin. Above these 
darker colored sandstones are more massive sandstones, weathering 
more or less whitish, which ascend at the north end of the cafion 
to perhaps three hundred and fifty feet above the stream bed. All 
vertebrate fossils that we found, of Permian age, were below these 
sandstones, which form a fairly definite horizon about the basin, 
and which may be taken as the lower limits of the Trias. 
It has been questioned by us elsewhere whether the vertebrate 
fossils found in Texas, Oklahoma, southern Kansas, Illinois, and 
Pennsylvania are really of Permian age. At the south side of the . 
cafion, the junior author found a perfect cast of a Spirifer, identified 
by Professor Schuchert as S. rockymontanus Marcou, a form occur- 
ring in Colorado in the Pennsylvanian. Though the specimen was 
found free, so that its exact horizon could not be determined, its 
excellent preservation proves conclusively that it had not been 
carried far from its original bed, and inasmuch as vertebrate 
fossils are found in the deepest strata of the cafion it seems quite 
certain that the specimen came from an intercalated bed among 
those yielding so-called Permian vertebrates. No other explanation 
seems possible. It is the conviction of both the present authors 
that the lowermost at least of the strata yielding vertebrate fossils 
are of Pennsylvanian age, and this conviction is strengthened by the 
known position of the vertebrate horizons in Texas, Kansas, 
