Island of Rodriguez.²

General description

Rodriguez is in the region of the E.S.E trade; which is usually strong, & a continuous current flows by it to the West. The main mass of the island is of basalt (of a similar character to that of Mauritius), which exhibits a strong tendency to break in the columnar form, and in one instance, in the valley of the Baie des Huîtres does so with great perfection, exhibiting a cliff of magnificent pentagonal pillars, 190 f^t. in height.

Remnants of upraised reef.

At the Eastern & western extremities of the Island however, are small districts of upraised coral.

The one to the S.W. is the largest, & is very curiously mingled with the igneous rock where they abut on one another, the layers showing that there must have been a second eruption, during which lava overlaid the coral beds.

In height this coral varies between 10 & 100 f^t. and the surface is sometimes even & water worn,—& sometimes broken into undulations & blocks.

All this upraised coral has been much exposed to water action, & is worn into undermined shapes, & is full of caverns, one of which is a contorted tunnel half a mile in length, apparently excavated by running fresh water, & not by action of the tide. Springs exist in it still, which run a short course & disappear into lower cavities.

The Eastern coral is very limited in extent, & is principally confined to the promontory of P^t. Coton. which is in course of demolition by the waves that at high tide have here considerable force, it being the weather side of the Island, & the fringing reef very narrow.

Small ledges of coral are to be found in all the Eastern valleys & the peculiar little bays called ‘Trous d’argent’³ are filled with upraised coral.

The inner side of P^t. Coton exhibits signs of the sea having once washed completely round the promontory, which has left perpendicular cliffs facing the East of the Island. The limestone here is remarkably white, compact & homogeneous, much resembling the Bermuda stone, & giving a metallic ring when struck.

Finally the Islands “Gombrani” “Pierrot” “Flat” & “Pianqui” are of coral, &, with the exception of a small mound on “Gombrani”, are quite level, & almost 10 f^t high.⁴

So much for the super marine coral.

Fringing reef.

The existing fringing reef is of the ordinary character but is remarkable by reason of its varying distance from the shore, ranging from a few yards on the weather side to 4 miles on the lee.⁵

It is living at all edges, & in most pools. It is abrupt on its outer edge, but in no case does it suddenly drop into more than 3 or 4 fathoms, the bottom sloping off to the level of the old & now submarine coral flat:

Submarine reef.

This submarine coral flat stretches off with a very uniform depth of about 35 fathoms of water on it, to a varying distance from the present reef of from 1$\frac{1}{2}$ to 12 miles, and thence falls suddenly into deep water.

I regret I had no time to carry a gradual line of soundings out from this edge, but I obtained some casts between Rodriguez & Mauritius, all of over 2000 fathoms, which proves the entire disconnection of the two Islands.⁶

The curious points to me in the distribution of coral are

(1) The absence of all upraised coral from the north & south sides of the present Island. especially from the former or lee side, where the wash of the sea is feeble.

(2) The insignificant growth of the present fringing reef on the weather side for surely the current cannot continually wash enough sand against the live coral to materially check its growth & the eternal swell ought on the contrary to encourage it.

W J L Wharton.