1036 
THE PHARMACEUTICAL JOURNAL A^D TRANSACTIONS. 
[J une 28, 1873. 
The Barrier Reef consists of a ridge of coral resembling 
that of an Atoll, but running parallel to the shores of a 
continent, or surrounding those of an ordinary island, and 
in either case at such a distance as to include between it 
and the land a deep channel of still water. 
A Fringing Reef differs from a Barrier Reef only in 
being smaller, and scarcely including any channel between 
it and the land, which it thus merely fringes with a skirt 
of coral rock. 
Mode of Formation of Reefs .—When it was first known 
that so many of the islands in the Pacific and Indian 
Oceans owed their existence to the energy of coral 
animals, it was believed that the coral-formers com¬ 
menced their labours upon the flat bottom of the sea, 
at unknown and unfathomable depths, and that they 
thence worked continuously upwards to the surface. 
This hypothesis, however, is inconsistent with the now 
well-established fact of the limited range in depth of 
living coral. 
The circular form of the Atoll suggested also another 
hypothesis, which maintained that the coral animals 
established themselves round the crater of some sub¬ 
marine volcano, and that, thence working upwards, they 
necessarily repeated on the surface of the sea the circular 
form of their foundation. Now this hypothesis would 
involve the highly improbable supposition that there 
exist throughout the region of corals innumerable 
volcanic craters, and that all these are nearly at the very 
same level, namely, that which alone is suited to the 
energies of the living coral ; while its improbability is 
rendered still further apparent by the fact that there is 
no known volcanic crater wdiose diameter approaches in 
dimensions to that of many Atolls. 
For the beautiful theory which is now universally 
accepted—the only one, indeed, which is consistent with 
all the known phenomena, and one of the most important 
with which the physical history of the earth has of late 
years been enriched—we are indebted to Mr. Darwin. 
The theory of Darwin is founded on two incontestable 
facts,—the one purely pysiological, the other purely 
physical. 
The physiological element in the theory of Darwin 
consists in the fact ali'eady insisted on, that the coral 
animals cannot live at unlimited depths, that they need 
the presence of light and of other conditions which they 
can obtain only near the surface, and that from twenty to 
thirty fathoms below the surface is the greatest depth at 
which the reef-building polypes can continue to live. 
But we have also seen that the outer walls of the coral 
reef have been followed downwards into depths very much 
greater than this ; and though the coral forming those 
deep portions is always found to be dead, it is plain that 
it must have been at one time living, and some additional 
fact is therefore needed in order to reconcile its occur¬ 
rence at these great depths with the established limited 
range of living corah Here, then, comes to our aid the 
physical element in the theory. It is this : That while 
the ocean maintains the same level from age to age all 
over the world, the solid land is subject to repeated oscil¬ 
lations of level, rising in one place and sinking in another, 
and this sometimes to an extent of many thousands of 
feet. 
Having demonstrated the reality of this phenomenon 
by reference to numerous well-known geological facts, 
such as the sinking of the southern shores of Scandinavia 
and of the western shores of G-reenland, and the rising of 
the northern shores of Scandinavia and of Siberia, the 
speaker proceeded, with the aid of diagrams, to apply, as 
Darwin had done, the two groups of phenomena, physio¬ 
logical and physical, to the explanation of coral for¬ 
mations. 
He showed how a mountain, rising out of the sea in 
the form of a precipitous island, in the region of the reef- 
builders, will present on its shores the conditions suited to 
the coral-polypes, which will there attach themselves, 
building downwards until they arrive at depths too great 
for the perfect exercise of their functions, and upwards 
until the surface of the sea sets bounds to the further 
elevation of their structures. A reef of coral will thus be 
spread all round the shores of the island, and will con¬ 
stitute the formation known as a Fringing Reef. 
But the island is supposed to be in a region of subsi¬ 
dence, and has begun to sink slowly into the sea, carrying 
with it the already-formed fringe of coral into depths in¬ 
compatible with the well-being of the polypes. Urged, 
however, by an unerring instinct, the reef-builders con¬ 
tinue their labours upwards simultaneously with the 
gradual depression of the land, and thus the reef is 
always extending itself towards the warm sunlit surface 
of the sea, where all the conditions of coral life exist, 
while the lower parts have passed downwards into depths 
v r here the formers of coral must cease to live. 
The reef has thus grown larger ; and as the coral is 
produced in greater force on the outer edge, where the 
reef is exposed to the open ocean, with all the conditions 
in which the animals forming it delight, this part is sooner 
brought to the surface than the inner part, where the 
growth of the coral is still further interfered 'with by the 
accumulation of fragments which the waves tear from the 
reef, break down into coral sand and coral mud, and carry 
inwards towards the land. A deep channel is thus formed 
between the outer part of the reef and the shores round 
which the coral has attached itself, and what was at first 
a Fringing Reef becomes in this way converted into a 
Barrier Reef. 
In the meantime fragments of coral broken off by the 
waves are gradually piled upon the upper surface of the 
reef, which is thus in time raised above the sea in the 
form of a long stretch of dry land, separated from what 
still remains of the original island by the intervening 
channel of still water, and capable in the course of time 
of affording, by the decomposition of its surface, a soil in 
which terrestial plants may take root. 
But the changes do not end with the formation of a 
Barrier Reef; for the work of subsidence goes on, and 
the ancient land continues to sink deeper and deeper into 
the sea, carrying the coral-polypes down vdth it into the 
dark, ungenial ocean depths, where they must inevitably 
perish. And now at last the highest point has disappeared, 
all has sunk beneath the sea, and a wide waste of land¬ 
less waters rolls unbroken over its summit. 
The island architects are not, however, to be baffled. 
As the lower parts of the reef sink into depths where 
they must perish, the upper parts are simultaneously 
extending themselves as a bank of living coral towards 
the surface, which at last they reach in the form of a 
more or less circular reef, on which the waves once more 
break, and which includes within it a sheltered lagoon, 
now free from even the last remnant of included land ; 
and the Barrier Reef becomes thus converted into an 
Atoll. 
Now it will be here noted that throughout the whole of 
the changes which have been thus traced, the thickness 
of the bed of living coral is a constant quantity, extend¬ 
ing always from a little below the surface of the sea to 
that fixed plane beyond which if the coral be carried it 
ceases to live, while, on the other hand, the mass of dead 
coral is continually increasing with the subsidence of the 
land. 
It is also evident from what has now been said that the 
Atoll points out the spot where an ordinary island had 
become submerged, and that the whole region of Atolls 
and Barrier Reefs has been gradually subsiding. A sub¬ 
ject of great significance is thus suggested by the study of 
the phenomena of Coral Reefs ; and the conclusion is 
irresistibly forced upon us that in the region where now 
the Pacific Ocean separates the Old World from the New 
there lay in former times a continent with its mountain 
peaks and table-lands ; that all has since sunk beneath 
the sea, except its highest table-lands and the summits of 
its highest mountains ; that within the region of the reef- 
builders the coral has gathered in its encircling reefs 
