Approaching this island on the northern or north-western side, a curved chain of bold mountains, surmounted by rugged pinnacles, is seen to rise from a smooth border of cultivated land, which gently slopes down to the coast.At the first glance, one is tempted to believe that the sea lately reached the base of these mountains, and upon examination, this view, at least with respect to the inferior parts of the border, is found to be perfectly correct.Several authors have described masses of upraised coral-rock round the greater part of the circumference of the island.(Captain Carmichael, in Hooker's "Bot.Misc." volume 2 page 301.Captain Lloyd has lately, in the "Proceedings of the Geological Society" (volume 3 page 317), described carefully some of these masses.In the "Voyage a l'Isle de France, par un Officier du Roi," many interesting facts are given on this subject.Consult also "Voyage aux Quatre Isles d'Afrique, par M.Bory St.
Vincent.") Between Tamarin Bay and the Great Black River I observed, in company with Captain Lloyd, two hillocks of coral-rock, formed in their lower part of hard calcareous sandstone, and in their upper of great blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of basalt; they were divided into beds dipping seaward, in one case at an angle of 8 degrees, and in the other at 18 degrees; they had a water-worn appearance, and they rose abruptly from a smooth surface, strewed with rolled debris of organic remains, to a height of about twenty feet.The Officier du Roi, in his most interesting tour in 1768 round the island, has described masses of upraised coral-rocks, still retaining that moat-like structure (see my "Coral Reefs") which is characteristic of the living reefs.On the coast northward of Port Louis, I found the lava concealed for a considerable space inland by a conglomerate of corals and shells, like those on the beach, but in parts consolidated by red ferruginous matter.M.
Bory St.Vincent has described similar calcareous beds over nearly the whole of the plain of Pamplemousses.Near Port Louis, when turning over some large stones, which lay in the bed of a stream at the head of a protected creek, and at the height of some yards above the level of spring tides, I found several shells of serpula still adhering to their under sides.
The jagged mountains near Port Louis rise to a height of between two and three thousand feet; they consist of strata of basalt, obscurely separated from each other by firmly aggregated beds of fragmentary matter; and they are intersected by a few vertical dikes.The basalt in some parts abounds with large crystals of augite and olivine, and is generally compact.The interior of the island forms a plain, raised probably about a thousand feet above the level of the sea, and composed of streams of lava which have flowed round and between the rugged basaltic mountains.These more recent lavas are also basaltic, but less compact, and some of them abound with feldspar, so that they even fuse into a pale coloured glass.On the banks of the Great River, a section is exposed nearly five hundred feet deep, worn through numerous thin sheets of the lava of this series, which are separated from each other by beds of scoriae.They seem to have been of subaerial formation, and to have flowed from several points of eruption on the central platform, of which the Piton du Milieu is said to be the principal one.There are also several volcanic cones, apparently of this modern period, round the circumference of the island, especially at the northern end, where they form separate islets.
The mountains composed of the more compact and crystalline basalt, form the main skeleton of the island.M.Bailly ("Voyage aux Terres Australes" tome 1 page 54.) states that they all "se developpent autour d'elle comme une ceinture d'immenses remparts, toutes affectant une pente plus ou moins enclinee vers le rivage de la mer; tandis, au contraire, que vers le centre de l'ile elles presentent une coupe abrupte, et souvent taillee a pic.
