_Tegion, now the great 
J. D. Dana on American Geological History. 319 
comparatively shallow seas, and at times emerging land; and 
was marked out in its great outlines even in the earliest Silurian. 
The same view is urged by De Verneuil, and appears now to be 
the prevailing opinion among American geologists. The depth 
. or may have been measured by the thousand feet, but not 
miles. 
TIL. During the first half of the lower Silurian era, the whole 
east and west were alike in being covered with the sea. In the 
first or Potsdam Period, the continent was just beneath or at the 
surface. In the next or Trenton Period, the depth was greater, 
iving purer waters for abundant marine life. Afterwards, the 
ast and West were in general widely diverse in their forma- 
tions; limestones, as Mr. Hall and the Professors Rogers have re- 
marked, were generally in progress over the West, that is, the 
Wissiec pi Valley, beyond the Appalach- 
ians, while sandstones and dials were as ge 
a great reef or sand-bank, partly hemming in a vast continental 
lagoon, where corals, encrinites and mollusks grew in profusion, 
thus aoe more or less perfectly the already existing At- 
lantic from the interior waters. 
IV. The oscillations or changes of level over the continent, 
through the Upper Silurian sid Devonian, had some reference 
to this border region of the continent: the formations approach 
or recede from it, and sometimes pass it, according to the limits 
of the oscillations eastward or westward. Along the course of 
the border itself there were deep subsidences in slow progress, 
48 1s shown by the thickness of the beds. It would require 
much detail to illustrate these points, and I leave them with this 
gar 
Onondaga Periods (the first two of the Upper Silurian) thin out 
i hee ch the Hudson River. Mr. 
the disturbances began as early, at least, as the close of 
wer Silurian, mentioning, too, that there is actually a 
of conformity at Gaspé between the beds of the Upper 
