THE ORIGIN OF CORAL REEFS. 55 
By this line of investigation, Professor Agassiz arrived at 
the conclusion that the coral reefs of these seas owe their 
form to agencies acting at and near the surface. The baneful 
action of sediment seemed to be amongst the more powerful 
of these agencies in determining the forms of reefs. The 
barrier-reefs are explained much in the way that Le Conte 
previously explained them. ‘The corals, unable to live in the 
muddy waters and on the muddy surface of the Florida Bank, 
fiourish in the clearer water of the steeper slopes beyond. 
Atolls owe their form to the action of the breakers and the 
currents, which, by driving the silt to leeward, cause the 
repression of the growth of coral except at the margins of 
the reef. 
We are now ina position to regard in one comprehensive 
view the explanations of Semper, Murray, and Agassiz con- 
cerning the forms of reefs. Though prominence is given by 
each of these three naturalists to particular agencies,—Semper 
dwelling on the currents and the tidal scour; Murray, on 
solution and the distribution of the food supply ; Agassiz, on 
the repressive influence of the sediment and the action of the 
breakers,—these differences arise from the circumstance of the 
problem being viewed and attacked from different stand- 
points. Wecannot doubt that we have in the views of these 
three observers an enumeration of the principal agencies that 
determine the forms of coral reefs. On the outer edge of the 
roof we have the directing influence of currents, the increased 
food-supply, and the action of the breakers. In the interior 
of the reef there are the repressive influence of sediment, the 
degradation of the dead coral by numerous boring organisms, 
the solution of the dead coral by the carbonic acid in the sea- 
water, and, lastly, the tidal scour. Few of those who have 
been engaged for any time in the examination of coral reefs 
can have any doubt as to the reality of these several agencies ; 
the only difference of opinion will be as to the relative im- 
portance they ascribe to each. These are the lines along which 
future observers will direct their researches. They are 
those, in fact, which have been followed in the more recent 
observations published since 1882. 
In a paper on the Recent Calcareous Formations of the 
Solomon Islands, which was published in the transactions of 
the Edinburgh Royal Society for 1885, I described my dis- 
coveries amongst the upraised coral reefs of this region. Here 
I found, as terrestrial formations, underlying the ancient 
reefs, the volcanic muds, the coral muds, the pteropod and 
globigerina oozes, and the. deep-sea clays of the Challenger 
Expedition, deposits that were originally formed in depths 
