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. Toutes ces montagnes sont formees de couches paralleles inclinees du centre de l’ile vers la mer.” These statements have been disputed, though not in detail, by M. Quoy, in the voyage of Freycinet. As far as my limited means of observation went, I found them perfectly correct. (M. Lesson, in his account of this island, in the “Voyage of the ‘Coquille’,” seems to follow M. Bailly’s views.) The mountains on the N.W. side of the island, which I examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and apparently another farther southward, have precisely the external shape and stratification described by M. Bailly. They form about a quarter of his girdle of ramparts. Although these mountains now stand quite detached, being separated from each other by breaches, even several miles in width, through which deluges of lava have flowed from the interior of the island; nevertheless, seeing their close general similarity, one must feel convinced that they originally formed parts of one continuous mass. Judging from the beautiful map of the Mauritius, published by the Admiralty from a French MS., there is a range of mountains (M. Bamboo) on the opposite side of the island, which correspond in height, relative position, and external form, with those just described. Whether the girdle was ever complete may well be doubted; but from M. Bailly’s statements, and my own observations, it may be safely concluded that mountains with precipitous inland flanks, and composed of strata dipping outwards, once extended round a considerable portion of the circumference of the island. The ring appears to have been oval and of vast size; its shorter axis, measured across from the inner sides of the mountains near Port Louis and those near Grand Port, being no less than thirteen geographical miles in length. M. Bailly boldly supposes that this enormous gulf, which has since been filled up to a great extent by streams of modern lava, was formed by the sinking in of the whole upper part of one great volcano.