On the
other hand, although most of the islands in the Pacific which
are encircled by barrier-reefs, are of volcanic origin, often
with the remnants of craters still distinguishable, not one of
them is known to have ever been in eruption. Hence in these
cases it would appear, that volcanos burst forth into action
and become extinguished on the same spots, accordingly as
elevatory or subsiding movements prevail there. Numberless
facts could be adduced to prove that upraised organic remains
are common wherever there are active volcanos; but until it
could be shown that in areas of subsidence, volcanos were
either absent or inactive, the inference, however probable in
itself, that their distribution depended on the rising or
falling of the earth's surface, would have been hazardous. But
now, I think, we may freely admit this important deduction.
Taking a final view of the map, and bearing in mind the
statements made with respect to the upraised organic remains,
we must feel astonished at the vastness of the areas, which
have suffered changes in level either downwards or upwards,
within a period not geologically remote. It would appear
also, that the elevatory and subsiding movements follow
nearly the same laws. Throughout the spaces interspersed
with atolls, where not a single peak of high land has been
left above the level of the sea, the sinking must have been
immense in amount. The sinking, moreover, whether continuous,
or recurrent with intervals sufficiently long for the
corals again to bring up their living edifices to the surface,
must necessarily have been extremely slow. This conclusion is
probably the most important one which can be deduced from the
study of coral formations; - and it is one which it is
difficult to imagine how otherwise could ever have been
arrived at. Nor can I quite pass over the probability of the
former existence of large archipelagoes of lofty islands,
where now only rings of coral-rock scarcely break the open
expanse of the sea, throwing some light on the distribution of
the inhabitants of the other high islands, now left standing
so immensely remote from each other in the midst of the
great oceans. The reef-constructing corals have indeed
reared and preserved wonderful memorials of the subterranean
oscillations of level; we see in each barrier-reef a
proof that the land has there subsided, and in each atoll a
monument over an island now lost. We may thus, like unto
a geologist who had lived his ten thousand years and kept a
record of the passing changes, gain some insight into the
great system by which the surface of this globe has been
broken up, and land and water interchanged.
[1] These Plants are described in the Annals of Nat. Hist.,
vol. i., 1838, p. 337.
[2] Holman's Travels, vol. iv. p. 378.
[3] Kotzebue's First Voyage, vol. iii. p. 155.
[4] The thirteen species belong to the following orders: - In
the Coleoptera, a minute Elater; Orthoptera, a Gryllus and a
Blatta; Hemiptera, one species; Homoptera, two; Neuroptera a
Chrysopa; Hymenoptera, two ants; Lepidoptera nocturna, a
Diopaea, and a Pterophorus (?); Diptera, two species.
[5] Kotzebue's First Voyage, vol. iii. p. 222.
[6] The large claws or pincers of some of these crabs are most
beautifully adapted, when drawn back, to form an operculum to
the shell, nearly as perfect as the proper one originally
belonging to the molluscous animal. I was assured, and as far as
my observations went I found it so, that certain species of the
hermit-crab always use certain species of shells.
[7] Some natives carried by Kotzebue to Kamtschatka collected
stones to take back to their country.
[8] See Proceedings of Zoological Society, 1832, p. 17.
[9] Tyerman and Bennett. Voyage, etc. vol. ii. p. 33.
[10] I exclude, of course, some soil which has been imported
here in vessels from Malacca and Java, and likewise, some small
fragments of pumice, drifted here by the waves. The one block of
greenstone, moreover, on the northern island must be excepted.
[11] These were first read before the Geological Society in May,
1837, and have since been developed in a separate volume on the
"Structure and Distribution of Coral Reefs."
[12] It is remarkable that Mr. Lyell, even in the first edition
of his "Principles of Geology," inferred that the amount of
subsidence in the Pacific must have exceeded that of elevation,
from the area of land being very small relatively to the agents
there tending to form it, namely, the growth of coral and
volcanic action.
[13] It has been highly satisfactory to me to find the following
passage in a pamphlet by Mr. Couthouy, one of the naturalists in
the great Antarctic Expedition of the United States: - "Having
personally examined a large number of coral-islands and resided
eight months among the volcanic class having shore and partially
encircling reefs. I may be permitted to state that my own
observations have impressed a conviction of the correctness of
the theory of Mr. Darwin." - The naturalists, however, of this
expedition differ with me on some points respecting coral
formations.
CHAPTER XXI
MAURITIUS TO ENGLAND
Mauritius, beautiful appearance of - Great crateriform ring of
Mountains - Hindoos - St. Helena - History of the changes in
the Vegetation - Cause of the extinction of Land-shells -
Ascension - Variation in the imported Rats - Volcanic Bombs -
Beds of Infusoria - Bahia - Brazil - Splendour of Tropical
Scenery - Pernambuco - Singular Reef - Slavery - Return to
England - Retrospect on our Voyage.
APRIL 29th. - In the morning we passed round the
northern end of Mauritius, or the Isle of France.
From this point of view the aspect of the island
equalled the expectations raised by the many well-known
descriptions of its beautiful scenery.
