82 Tin American Geologist. February, 1894 
the residual soil on surfaces two or three hundred feet lower. 
The differences in condition and appearance indicate enor- 
mous differences in the length of time the specimens have 
been exposed. The effects on the better preserved specimen 
of rain and frost and wind-driven sands, with frequent falls 
from undermining cliffs, during the years necessary to reduce 
the hill on which it lies to the level now occupied by the 
fragments with which it is compared, will not be left to 
conjecture so long as the worn and dismembered fragments 
lying at lower levels remain to furnish objective illustrations 
of what those effects have been in the past. There are rea- 
sons for the conclusion that all the silicified trunks, including 
those of Bennettites, came from the same horizon and that 
that horizon was the vitrified beds near the summit of the 
Dakota sandstone. 
East of the valley followed in this vicinity by the B. 
& M. railway, rises Arnold's peak, a high butte, the sum- 
mit eight hundred feet above the valley, and. like the 
Other high points of the region, capped with vitrified sand- 
stone. The geological structure at the base is concealed, but 
a mile or two farther north, almost directly east of Minne- 
kahta. the ridge of which Arnold's peak is simply the most 
prominent part, reveals at its base the belemnite-bearing 
beds of the Jurassic. The plain on which Minnekahta 
stands is some scores of feet below the top of the Jurassic 
and not less than six hundred feet below the vitrified sand- 
stone near the summit of the Dakota group. On this plain a 
few specimens of Bennettites have been found, but in most 
cases they were so far decomposed as to fall to pieces when 
attempts were made to remove them. Again we find some 
relation between the abrasion and decomposition that the 
fossils have undergone and the vertical distance they lie be- 
neath the vitrified beds. Assuming that all the fossils were im- 
bedded at essentially the same horizon, then those that now 
occupy the lowest level have been longest exposed to atmos- 
pheric and aqueous agencies. 
At Hot Springs, — about twenty miles, as one has to travel, 
from the principal group of cycads, — the valley of Fall river 
has been CUt down through the entire thickness of the Dakota 
sand-tone, through all the Jurassic, and down into the pur- 
