DARWIN MEMORIAL CELEBRATION 
25 
Hall would have enabled him to extend his list of extinct forms common to 
both continents; and possibly he might have anticipated some of the all- 
important generalizations for which the world is indebted to the former 
president of this academy who now is president of the museum. 
Nothing in South America, east or west, escaped Darwin; from glaciers 
to peat bogs, from earthquakes to climatal variations, everything was im¬ 
portant; but what impressed him most on both sides of the continent were 
the evidences of extremely slow secular movement in the earth’s crust. 
This was the preparation for that study of the coral islands which resulted 
in his chief contribution to philosophical geology. 
Many voyagers prior to 1833 had observed and had tried to account 
for the strange atolls, or low ring like coral reefs, each inclosing a lagoon 
which communicates with the sea by a narrow channel; but Darwin dis¬ 
covered other forms of reefs which were equally perplexing. Many islets 
of rock are fringed by coral growth, while vast barrier reefs, separated from 
the land by channels of varying depth, extend at times for hundreds of miles 
along coasts. All explanations by previous observers were defective, as they 
seemed to ignore these types as well as other features, not less important. 
Reef-making corals can not endure exposure to the air and they can 
not thrive at a depth of more than 20 fathoms, so that their vertical range 
is about 115 feet; yet hooks and anchors brought up coral rock and sand 
from many hundreds of feet below the limit of growth; in a great number 
of instances, the atolls or ring-like reefs are mere peaks rising with abrupt 
slopes from “fathomless” abysses. Coral-bearing areas within the Indian 
and Pacific Oceans are of vast extent, there being chains of archipelagos 
1,000 to 1,500 miles long. The reefs are rudely circular or elliptical in the 
islands, but are linear along the coasts; in the one case, the reef incloses a 
lagoon, in the other, a lagoon-like channel separates the reef from the coast. 
These are fundamental elements of the problem, not one of which may be 
neglected in the solution. A clue to the explanation was found by this keen 
observer, when he saw an islet of old rock, fringed with coral, rising from the 
lagoon of an atoll, so that the atoll-ring resembled in many respects the 
barrier reef of a continent and the lagoon itself resembled the lagoon-like 
channel seen on the Australian and other coasts. 
Chamisso’s suggestion that coral reefs had been formed on banks of 
sedimentary material seemed wholly incompetent to meet the conditions, 
for the areas are too vast, and Darwin was compelled to believe that the 
atolls rest on rocky bases; but even on this supposition, it appears incredible 
that peaks of several great mountain chains should all come to within less 
than 180 feet of the surface and that not one rose any higher. The long 
study in South America had prepared him to seek an explanation in mobility 
