212 Claypole on (Darwin and Geology. 
ground on which at last it came to rest. It is not our duty here 
to follow the story. Its results were mainly biological, though 
the indirect results to geology were important. 
The earliest of these was his theory of coral-reefs on which, 
in view of some recent discussions a few words will be in 
place. 
The formation of coral-reefs and coral-islands had lone: been 
an unsolved problem. The opinion entertained by most writ- 
ers on the subject had been that these islands and reefs incrust- 
ed the tops of submarine mountains, and this was adopted and 
advocated by Lyell in the earlier editions of his "Principles." 
Their great number and size however, militated strongly 
against the theory; but it held its ground in default of anything 
more satisfactory. 
From some observations made during his voyage Mr. Dar- 
win was led to propose another hypothesis. He suggested that 
coral-islets of all kinds were based on the tops of slowly subsid- 
ing mountains which formerly reached or rose above the sur- 
face. On this view the polyps began their work when the peak 
w^as not more than 120 feet under water, and by the constant 
secretion of limestone maintained it at that level, subsidence 
notwithstanding. As the growth of coral is most rapid in the 
open sea so the outer edge of the encircling reef grew fastest 
and rose, while the inner parts, being shut oif from the free 
food and abundant calcareous supply of the open water, flour- 
ished less and ultimately died. In this way as the land sank 
and became smaller the reef became more distant and all the 
various stages between the circling line of coral and the per- 
fect atoll were developed, or in the case of a large tract of land 
such as Australia, the great bafrier reef that borders its north- 
eastern coast for about a thousand miles, was the result. 
Such was the simple and beautiful solution proposed by Dar- 
win for a problem which had been previously unsolved. Its 
simplicity and beauty w^on for it immediate popularity and it 
remained for many years almost undisputed. 
The vastness of the main postulate on which Darwin based 
5^ 
which he made in 1842 at the age of 33 he says: " This was the last time 
I was ever strong enough to climb mountains or take long walks such as 
are necessary for geological work.' 
