84 J. D. Dana-—— Oceanic Coral Island Subsidence. 
the continent had been made and raised to their full height, 
and the surface crust thickened over all the continent, except 
that of the Azoic nucleus, by successive beds to a thickness of 
thousands of feet, even thirty-five thousand by the close of the 
Paleozoic along the Appalachians, and probably much beyond 
this on the Pacific border; and-when these thick sediments had 
in many regions been stiffened by crystallization or metamor- 
hism; I say it is reasonable that, finally, changes of level, 
through the working of the old system of forces, should again 
have affected most the old nucleal Azoic area of the continent, 
where there had been no thickening except what had taken 
a internally ; and that, if one arm of the V, that along the 
anadian watershed, were raised at this time—as the facts 
prove—the other, northwestern in trend, should also have been 
raised, and to a greaterextent. This is at least probable enough 
to become a question for special examination over the region. 
These northern continental upward movements which intro- 
duced the Glacial era, carrying the Arctic far toward the 
tropics, may have been a balance to the downward oceanic 
movements that resulted in the formation of the Pacific atolls. 
hile the crust was arching upward over the former (not ris- 
ing into mountains, but simply arching upward), it may have 
been bending downward over the vast central area of the great 
n. 
The changes which took place, cotemporaneously, in the 
Atlantic tropics, are very imperfectly recorded. The Bahamas 
show by their form and position that they cover a submerged 
land of large area stretching over six hundred miles from north- 
west to southeast. The long line of reefs and the Florida Keys, 
trending far awa i 
dence that this Florida region participated in the downward 
banks, and also the blankness of the ocean’s surface, all appear 
to bear evidence to a great subsidence. 
The peninsula of Florida, Cuba, and the Bahamas look, as 
they lie together, as if all were once part of a greater Florida or 
southeastern prolongation of the continent. The northwestern 
and southwestern trends, characterizing the great features of the 
American continent, run through the whole like a w and 
woof structure, binding them together in one system ; the former 
trend, the northwest, existing in Florida and the Bahamas, and 
