of Edinburgh, Session 1885 - 86 . 
893 
upheaved coral rocks thin away as one ascends the slopes of these 
islands, an idea may he formed of the rapidity with which dead 
coral may he removed in solution hy the carbonic acid in rain water. 
The extensive deposits of calcareous tufa, that in crust the sides of 
the streams and form step-like ledges in their courses, have been 
derived originally from the solution of these rocks. The surfaces 
of the elevated reef-flats of St Christoval and other islands are honey- 
combed in a remarkable manner by the atmospheric agencies ; and in 
places where the rock is exposed to the dripping from overhanging 
cliffs, it is worn into holes, 12 to 18 inches deep, containing water. 
In a similar manner the carbonic acid contained in sea-water 
acts on dead coral ; but of the extent of its action and of its 
ultimate effect I was unable to obtain the same direct evidence. It 
is to this agency that Mr Murray mainly attributes the formation of 
lagoons and lagoon-channels. As bearing on this subject, I append 
the following remarks : — 
In the interior of Oima atoll I noticed some large masses of living 
coral which presented the characters of miniature atolls. One large 
circular mass of Mad.repora (with short branches), which measured 
18 feet across and 2 feet in height, possessed a dead centre that was 
depressed 9 or 10 inches below the level of its living margin. In 
this basin at “ low water springs ” there were a few inches of water 
forming a miniature lagoon ; whilst the living margin was exposed 
for several inches above the surface. In the lagoon sea-urchins, 
star-fish, sea-anemones, stalked mushroom-corals, and cowries were 
to be seen in the crevices ; tiny fish darted about from one side to 
the other ; while a beautiful blue Botryllian and pink Nulliporce, 
partially concealed the unsightliness of the dead coral. From the 
manner in which the dead branches of the Madrepora in its 
central basin were blunted and rounded off, it was evident that, as 
the mass grew outwards, it had died and been subsequently dis- 
solved away in its centre. An adjacent flat-topped mass of Porites , 
measuring 13 feet across, at the same condition of the tide presented 
an example of another miniature atoll. Its central portion was 
dead and hollowed out into a basin, which was occupied by a small 
pool of water ; whilst its living border was exposed for a few inches 
above the surface. In the pool grew tiny lumps of a Pocillopora 
and some Alcyonarians. 
