132. Mr. Dana on Areas of Subsidence in the Pacific, 
as that of the Pacific and East Indies, may well be receixed with 
some hesitation. According to my own observations, regions in — 
which his theory would require a subsidence, have actually expe- 
rienced an elevation at some recent period. I might instance 
several examples of this elevation in various parts of the Pacific. 
Suffice it to say here, that I found nothing to support the princi- 
ple laid down by him, that islands with a barrier reef are subsi- 
ding, while those with only a fringing reef are rising ; indeed 
facts most stubbornly deny it. Without entering upon the dis- 
cussion of these facts, which, as they will appear in the Govern- 
ment publications, I am not at liberty to dwell upon here, I pro- 
pose to point out what are the regions of subsidence which the 
coral islands in the Pacific indicate as having been in progress 
during their formation. 
Before proceeding, I may be excused for adding here a few 
words in explanation of Mr. Darwin’s theory with regard to the 
formation of coral islands. He rejects the unfounded hypothesis _ 
that coral islands are built upon the craters of extinct volcanoes, -— 
and proposes the following theory in its stead, which is supported 
by a minute as well as general survey of the facts. 
The coral belt or atoll, he supposes to have been originally a 
barrier reef around a high island, like the reef around many is- 
lands in the Pacific. When the reef commenced, it could not 
have been extended to a lower depth than one hundred or one 
hundred and twenty feet, for this is the limit of the reef-forming 
corals. But if the island gradually subsided—so gradually that 
the corals could by their growth, keep themselves at the surface, 
the reef might finally attain any thickness, according to the ex- 
tent of the subsidence. In this manner, subsidence might finally 
submefge the whole island, and leave nothing but the reef at the 
surface. Mr. Darwin points to instances in which only the 
mountain tops now remain above the ocean. Carry the process 
a little farther, and we have the coral belt surrounding its little 
sea—the usual condition of the coral island. 
This theory, as is seen, supposes extensive subsidence. And 
so we remark must every theory: for without it, we could only 
have reefs one hundred and twenty feet in depth, instead of the 
great thickness they are believed to possess. It is my present . 
object to fix the area of this subsidence, and suggest something 
with regard to the extent of it in different parts of the ocean. 
ene iene 
—a s 
