“ 
Miscellaneous. 487 
barrier reefs surrounding volcanic islands, and passing gradually 
towards the south into coast reefs, which are closely applied to coral 
reefs elevated to as much as 200 feet. The group of islands termi- 
nates in a small island entirely destitute of reefs, separated from 
the rest by a channel four geographical miles in breadth. If we 
were to apply Darwin’s theory to this example, we should have to re- 
gard this southernmost island as a resting point, whilst the northern 
-part, by sinking, had rendered the formation of atolls possible. In-— 
dependent of the improbability that a mountain-chain ascending from 
“the deep sea at a distance from all other insular groups, and having 
so small a horizontal extension (sixty miles in length by six to seven 
miles in breadth), should possess so great a difference of vertical 
‘movement, facts directly observed testify against a depression, 
nay, even render it probable that an elevation has taken place in 
recent times. The northern volcanic islands are formed by two dif- 
ferent basaltic eruptions, one of which bears the present and the 
older elevated reefs of the south, whilst the second partially overlies 
them. Traces of a trachytic eruption also occur, but, apparently, 
isolated from the larger basaltic island, These islands, therefore, 
-belong to a comparatively very recent geological period, And the 
elevated coral reefs of the south, partly converted into dense coral- 
line limestone, in other places decomposed into chalk, pass directly 
over into the existing reefs. A depression is further disproved by the 
nature of the submarine surface in the interior of the lagoon-chan- 
‘nel. Whilst in the north there is a deep and often very broad 
channel which separates the outer reef from the shore of the island, 
the numerous small elevated coralline limestone islands of the south 
are united by a surface extending for many miles nearly horizontally 
.searcely 4—6 fathoms beneath the surface of the sea, and which, in 
~ still water at the time of the spring tides, may be traced out of the 
sea into the supramarine rocks and islands. A horizontal surface 
‘attaining such colossal dimensions could not possibly be formed 
during a depression which, a few miles further north, had produced 
a channel of 70 fathoms in depth. 
The author rather regards the physical influences, especially the 
internal sea-currents caused by the rain, and the exterior direct and 
diverted ones, as the causes which have produced in the north the 
atolls, and in the south the coast reefs, simultaneously with an 
elevation. Whilst in the latter the deep-going eroding action of the 
wave-blow or the wash of the sea has gradually planed away the 
dense and solid coralline limestone to a nearly horizontal surface, 
which lies at about the depth to which the sea-wash is capable of 
acting, in the north the becks coming down from the mountains, 
conjointly with the wash and currents of the sea, have acted much 
‘more strongly upon the uncommonly soft, readily decomposable 
basalt of the west, than was the case with the limestone in the 
south, and have eaten out the deep lagoon-channels, which in par- 
| _ ticular places extend to the width of a mile between the solid 
ground and the outer reef.—Verhandl. der phys.-med. Gesellsch. in. 
Wurzburg, February 1, 1868. | 
