# Memoir on the Island of Navassa (West Indies)/Elevated Coral Islands

Layout 2
III.—Elevated Coral Islands.
Having thus described the form of the Island of Navassa, and the character of the rock which constitutes its whole frame, in comparing it with other upraised coral Islands, my object is not only to remove all possible doubt as to its origin, but from identity of forms, general likeness and peculiarities, to arrive to some conclusions of economical importance for its "exploitation."
In his book on coral reefs and Islands, our celebrated geolo- ​gist, James D. Dana, gives the following description of Metia or Aurora Island, one of the Western Paumotas group in the Pacific Ocean.
"It is a small Island about four miles by two and a half in width, and 250 feet in height, and consists throughout of coral limestone. On approaching it from the north-east, its high vertical cliffs were supposed to be basaltic, and had much resemblance to the palisades of the Hudson. This appearance of a vertical structure was afterwards traced to vertical furrowings by the waters dripping down its front and the con-sequent formation of stalagmitic incrustations. Deep caverns were also seen.
"The cliff, though vertical in some places, is roughly sloping in others, and on the west side the surface of the Island gradually declines to the sea.
"The rock was found to be a white and solid limestone, seldom presenting any traces of its coral origin.
"In some few layers there were disseminated corals looking like imbedded fossils along with beautiful casts of shells, but for the most part it was as compact as any secondary marble, and as uniform in texture. Occasionally there were disseminated spots of crystallized calc-spar.
"The surface of the Island is singularly rough, owing to the erosion of rains.
The shores at the first elevation of the Island must have been worn away by the sea, and the cliff and some isolated pinnacles of coral rock still standing on the west are evidence of degradation. But at present there is a wide shore platform of coral reef, 200 to 250 feet wide, resembling that of the low coral Islands, and having growing coral, as usual, about the margin and in the shallow depth beyond."
After this description, showing such a striking resemblance between Aurora and Navassa, amongst several other similar Islands met in the Pacific, it will be sufficient for my object to quote the following:
Mangaia, nearly 300 feet high, has a level summit, and is surrounded by a terrace like plain, at about the height of 100 feet. It is formed of coral rock with some fine ground basalt ​in the interior. In the central hollow of the upraised reef there are many scattered patches of coral rock.
Rurutu has an elevated coral reef 150 feet in height.
Vavau, the northern of the group of the Tonga Islands, made of coral limestone, 30 to 100 feet in height, has precipitous cliffs with many excavations along the coast.
Savage Island, a little to the cast of the Tonga group, resembles Vavau in its constitution and cavernous cliffs, and is elevated 100 feet.
The cliffs of Elizabeth Island are 80 feet high, and from the description of Capt. Beechey, appear to consist of a homogeneous coral rock.
From the above quotations it will be seen that like Navassa, those other elevated coral Islands most generally do not exceed 300 feet in height.
Like at Navassa the cliffs, vertical in some parts, slope roughly in others, and on one side of the Island, gradually decline to the sea.
The rock, a white and solid limestone, seldom presenting any traces of its coral origin, still contains accidentally imbedded corals.
Like at Navassa, the surface of those Islands is singularly rough.
Finally Navassa, like the other elevated coral Islands, presents this most singular peculiarity, the shore platform, rising above the level of the sea as a vertical cliff; at Navassa, from fifteen to sixty feet high, till on the Northern side of the Island it gradually declines to the sea, forming a low rocky beach.
The deep indentation that I have described at the foot of the lower bluff of Navassa, is a fact in confirmation of the theory of J. D. Dana, on the formation of the shore platform of the elevated coral Islands, when he says that the existence of this platform, apparently so peculiar, is due to the simple action of the sea, and is a necessary result of this action. Passing to New Holland from the coral Islands of the tropics, he found there the same structure exemplified along the sandstone cliffs, in most places one hundred or more feet in height. There is a layer of sandstone rock lying like the shore platform of a coral Island near low tide level, and from fifty to one hundred and ​fifty yards in width. It is continuous with the bottom layer of the cliff, the rocks which once covered it having been removed by the sea.
The erosion of the foot of the shore platform bluff at Navassa, and the enlargement of the fringing reef, cornice like, now building by living corals, would reproduce the same phenomenon, if a new elevation of the Island would take place; but as it is, its incipient form seems to be a very strong argument in favor of Dana's theory of the origin of the shore platform.
The Island of Navassa, as we have demonstrated by its lithoological characters, and resemblance of form with other similar Islands, being an elevated coral Island; with Darwin, we must admit that to rise to its actual height it had to subside and be elevated several times, as it is a fact well known that corals do not build above the level of the sea.
I did not remain at Navassa a sufficient length of time to allow me to make the minute observations that would be necessary to ascertain the different periods of elevation and subsidence, but some old air-holes, found below the summit of the Upper Cliff on the north side of the Island show plainly that the rocks now at an elevation of about 300 feet, were once on a level with the sea, and probably records of the same kind would be found lower down if the rocks, that have tumbled down against the Upper Cliff, were removed.
Also the works now in progress at Navassa, to remove the Phosphate, will probably, in the course of time, show the differences existing between the strata built at different periods; although to-day all the surface rock of the Island seems to be homogeneous by its long exposure to the same atmospheric influences. The regularity and horizontality of the stratification at Navassa also precludes the supposition of any volcanic action in the elevation of the Island, or of any violent convulsion during the earthquakes which have so often desolated the two neighboring Islands of Jamaica and Hayti. The absence of all traces of extraordinary disturbance would lead us to admit periods of gradual subsidence and elevation, which will require for demonstration a more complete study of the geology of the West Indies Islands.
​Having arrived to the conclusion that Navassa is an elevated coral Island, I had to examine the question, if in such formation there was hope to find a supply of fresh water for the use of the officers and operatives of the Company. Up to the present time the necessary water had to be imported, or distilled at great cost from the sea water.
The existence on the south side of the Island of a cave containing, after rain, a beautiful sheet of fresh water, and called by the operatives the "spring" cave, gave me the hope to find a place where I would be justified, with some chance of success, to advise the boring of a well. But the horizontality of the stratification, the formation of the limestone full of irregular cavities of all sizes and shapes, and my remark, during an excursion of examination in a row boat around the Island, that there was a stratum very near the surface of the sea, from which the water dropped in many places, leaving dark spots, showing the spongious character of the rock, left me little hope of the existence of an underground water level of sufficient importance for any economical purpose.
After those observations I felt disposed to abandon entirely the idea of boring for fresh water, and merely advised to turn into a cistern, by some masonry work, the spring cave, in view of saving all the rain water falling through this opening to the sea, a quantity sometimes considerable after heavy rains. But with every step that I made towards the conclusion, that Navassa was a coral Island, I felt that I should not give up the hope of finding fresh water, as we may expect to find the same phenomena in identical geological formations.
Remark.
While reading the proof-sheets of this memoir, I have received a very elaborate paper on the geology of the Key of Sombrero, West Indies, by Alexis A. Julien, Esq., Assistant in the School of Mines, Columbia College, published in the Annals of the Lyceum of Natural History, of New York, Vol. viii, April, 1866, Nos. 8, 9 and 10. The many analogies between the geological formation and deposits of Phosphatic Guano of Navassa and Sombrero are very remarkable. I entertain little doubt that the interesting observations of Mr. J. on the successive periods of subsidence and elevation at Sombrero, will find new and strong arguments in favor of his scientific views when the development of the diggings, or more complete observations than mine, will allow a more complete discrimination between the different strata of Navassa.