Coral Reefs

Page 77

This madreporitic formation is mentioned by M. Cordier in his report to the Institute (May 4th, 1839), on the voyage of the "Chevrette", as one of immense extent, and belonging to the latest tertiary period.) as being very low, and consisting of a limestone with shells and corals of very recent origin; he adds, that it does not admit of a doubt that the sea has retired from this district even within the memory of man. There is also some reason for believing that the western shores of India, north of Ceylon, have been upraised within the recent period. (Dr. Benza, in his "Journey through the N. Circars" (the "Madras Lit. and Scient. Journ." volume v.) has described a formation with recent fresh-water and marine shells, occurring at the distance of three or four miles from the present shore. Dr. Benza, in conversation with me, attributed their position to a rise of the land. Dr. Malcolmson, however (and there cannot be a higher authority on the geology of India) informs me that he suspects that these beds may have been formed by the mere action of the waves and currents accumulating sediment. From analogy I should much incline to Dr. Benza's opinion.) MAURITIUS has certainly been upraised within the recent period, as I have stated in the chapter on fringing-reefs. The northern extremity of MADAGASCAR is described by Captain Owen (Owen's "Africa," volume ii., page 37, for Madagascar; and for S. Africa, volume i., pages 412 and 426. Lieutenant Boteler's narrative contains fuller particulars regarding the coral-rock, volume i., page 174, and volume ii., pages 41 and 54. See also Ruschenberger's "Voyage round the World," volume i., page 60.) as formed of madreporitic rock, as likewise are the shores and outlying islands along an immense space of EASTERN AFRICA, from a little north of the equator for nine hundred miles southward. Nothing can be more vague than the expression "madreporitic rock;" but at the same time it is, I think, scarcely possible to look at the chart of the linear islets, which rise to a greater height than can be accounted for by the growth of coral, in front of the coast, from the equator to 2 deg S., without feeling convinced that a line of fringing-reefs has been elevated at a period so recent, that no great changes have since taken place on the surface of this part of the globe. Some, also, of the higher islands of madreporitic rock on this coast, for instance Pemba, have very singular forms, which seem to show the combined effect of the growth of coral round submerged banks, and their subsequent upheaval. Dr. Allan informs me that he never observed any elevated organic remains on the SEYCHELLES, which come under our fringed class.

The nature of the formations round the shores of the RED SEA, as described by several authors, shows that the whole of this large area has been elevated within a very recent tertiary epoch. A part of this space in the appended map, is coloured blue, indicating the presence of barrier-reefs: on which circumstance I shall presently make some remarks. Ruppell (Ruppell, "Reise in Abyssinien," Band i., s. 141.) states that the tertiary formation, of which he has examined the organic remains, forms a fringe along the shores with a uniform height of from thirty and forty feet from the mouth of the Gulf of Suez to about latitude 26 deg; but that south of 26 deg, the beds attain only the height of from twelve to fifteen feet. This, however, can hardly be quite accurate; although possibly there may be a decrease in the elevation of the shores in the middle parts of the Red Sea, for Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran Island (latitude 15 deg 30' S.) shells and corals, apparently recent, at a height between thirty and forty feet; and Mr. Salt ("Travels in Abyssinia") describes a similar formation a little southward on the opposite shore at Amphila. Moreover, near the mouth of the Gulf of Suez, although on the coast opposite to that on which Dr. Ruppell says that the modern beds attain a height of only thirty to forty feet, Mr.

Charles Darwin

All Pages of This Book