VIEWS OF PROFESSOR DANA. 
311 
one of the strongest evidences of subsidence all the world 
over, he quotes the Marquesas Islands with the Gambier 
and Hogolen Islands, Raiatea and Bolabola of the Tahiti 
Group and the Exploring Isles of the Feejees. Professor 
Dana also calls attention to the general parallelism between 
the average trends of coral islands and the courses, not 
only of the groups of which they form part, but also of the 
groups of high islands not far distant, 1 * * and refers to the 
arguments drawn by Mr. Darwin from the fact that the 
larger coral islands have the same diversity of form as 
is found in the barrier-reefs of high islands and exhibit 
groupings such as would result from the sinking of a large 
island of ridges and peaks with encircling reefs. The 
depth of the lagoon, and of the channels inside of barrier 
reefs — in many cases two or three times greater than 
twenty fathoms — is very difficult to explain if there has 
been no subsidence ; so is that of the ocean near to atolls. 
Professor Dana, after noticing one or two considerations 
of a general nature, points out that 4 if an atoll reef is not 
undergoing subsidence, the coral and shell material pro- 
duced which is not swept away and distributed by currents 
serves: (1) to widen the reef; (2) to steepen, as a conse- 
quence of the widening, the upper parts of the submarine 
slopes; (3) to accumulate, on the reef, material for beaches 
and dry land ; and (4) to fill the lagoon. But if, while 
subsidence is in progress, the contributions from corals 
and shells barely compensate for the loss by subsidence 
and current waste, the atoll-reef, unable to supply suffi- 
cient d&bris to raise the reef above tide level by making 
beaches and dry land accumulations, would (1) remain 
mostly a bare tide-washed reef; (2) lose in diameter or 
size because the debris that is not used to keep the reef at 
1 This, however, I conceive, would not offer a difficulty to those 
who advocate submarine volcanic masses as a foundation for the 
reefs. 
