234 The American Geologist April, 1892 
rampart through which the streams have cut narrow channels, by 
which entrance is with difficulty gained to the central fastness. 
A fine illustration of this structure is seen along the narrow- 
gauge railway running up Elk creek from Postville to Lead City. 
Though the Jurassic beds that follow are conformable to the 
Trias, yet no greater contrast can well be imagined than that 
offered by these two formations. The former is absolutely desti- 
tute of all trace of life, while the latter contains some of the 
richest fossiliferous strata known anywhere on earth. What 
physical changes are indicated by this difference cannot now or 
here be discussed, but whatever they were, they permitted the 
abounding reptilian life of the Jurassic to invade the lifeless seas 
of the Trias. The rapid thickening of the former beds to the 
northwest, as pointed out by Prof. Carpenter and others, shows 
that the land whence they came lay in that direction, while the 
gypsum of the Trias would imply a closed sea and great evapor- 
ation. 
The vast and varied reptile fauna now accumulated by Prof. O. 
C. Marsh, at New Haven, has been chiefly taken from the Juras- 
sic of the region surrounding the Black Hills. Many of its 
members being in part or altogether terrestrial, indicate land at 
no great distance, and the Hills then probably rose as islands 
above the Jurassic waters. 
Of this fact there is no room for doubt at the commencement 
of the Cretaceous era, for the fine conglomerate at its base owes 
its origin to the Pre-Cambrian strata of the axis. But the emer- 
gence was apparently only temporary, and was followed by sub- 
sidence, which again carried the Hills under water, and allowed 
the deposition of the later Cretaceous beds over the whole area. 
These strata indicate marine conditions, but no great distance 
from land, as they consist of clays and shale, with little lime, and 
contain abundant and beautiful specimens of Baculites, Am- 
monites, ete. 
IX. 
It seems probable that the Cretaceous sea continued on into the 
Kocene era, and that Eocene strata were deposited over the Cre- 
taceous. As Prof. Carpenter has shown, there is evidently a gap 
in the record here. The Eocene is wanting in the Black Hills, 
and the Miocene lies immediately on the Cretaceous. During 
some part of this interval the central peaks, if not more, served 
‘ 
a 
