Current Action in the Ordorician. — Ruedemann. 391 
The cause of the extension of the Trenton limestone all 
over the continent, even over the eastern border region, has 
been sought in the existence of a ''barrier region outside of 
the limestone area, near or outside of the present Atlantic 
coast line."* Billingsf also explains the prevention of Brit- 
ish specimens from being introduced into the Trenton sea by 
the presence of this eastern coast barrier; which latter dipped 
down beneath the ocean in the following period and allowed 
the incursion of British species, which he observed. Geolo- 
gists, in general, have resorted to the assumption of the for- 
mer existence of this land in order to account for the enor- 
mous accumulation of sediments in the Appalachian region, 
during the Paleozoic age. The writer sees in the observation 
of the arranged fossils a new argument in favor of this theory, 
for the direction from which the current comes miist also im- 
plicitly indicate where, in the- preceding Trenton period the 
barrier was, that closed the Appalachian basin towards the 
Atlantic ocean. As the transition from the Trenton limestone 
to the Utica shale in the east, and to the Galena limestone in 
the west, takes place only gradually, by a hundredfold alter- 
nation of tlie relative rocks, it must be concluded that there 
has been a long period of oscillations of the barriere, these 
oscillations alternatingly closing and opening the continental 
basin to the Atlantic current in the northeast. 
As the limestone continued to form in Anticosti during the 
Hudson River epoch (including the Utica shale) and only thin 
beds of shale are reported to occur in the series, it impossible 
that this region already lay outside of the shallowJ^ea and of 
the archipelago which Dana supposes to have occupied the 
site of the present Appalachian region and Green mountain 
system, and which, under the wearing action of the sea waves 
must be supposed to have furnished part of the fine mud con- 
solidated into the Utica slate and shale. 
TQjwards the end of the Hudson period, the uplift of the 
Green mountains and the emergence of the Appalachian folds 
probably turned the Labrador current again into the Atlantic 
ocean. 
*Dana, op. cit., p. 208. 
tCf. Dana, op. cit., p. 250. 
