# Memoir on the Island of Navassa (West Indies)/Deposits of Phosphates

Layout 2
V.—Deposits of Phosphates.
All the cavities of the coral rock forming the frame work of the Island of Navassa are filled by deposits of Phosphate of lime.
Those cavities are of every form and size, their opening and depth varying from a few inches to several feet. I cannot convey a better idea of this formation than by saying that it resembles an immense group of coral branches, in which all the interstices have been filled by those deposits.
Their origin explains itself by the presence on the Island of innumerable quantities of sea birds, Boobies and Frigates, and a species of large lizards, the Iguanas, not less numerous.
The dung and bones of those birds and lizards accumulated for centuries have formed those immense deposits.
In any excursions on the Island, I never stopped at any place without finding a quantity of debris of bones of those animals; sometimes collecting from a square yard a handful of pieces of bones. As it should be expected, the last debris generally consists of the hardest parts of the bones of the Iguanas, such as the maxillary, and the large bones of the birds; the small bones having been first decomposed. How from such sources those immense deposits have been formed, may be accounted for by the circumstance that there is on the Island no animal of genera, carnivora or rodentia, few insects, and consequently no cause for the destruction of bones.
Their dissolution progresses under the most favorable conditions, accelerated by the fermentation of the dung, which is rapid in a hot climate, under the alternating influence of the heavy dews of the night and the intense heat of the day.
The putrefaction which takes place causes copious and continuous emanations of carbonic acid, and since it has been ascertained by numerous experiments that the solubility of ​phosphate of lime depends upon the presence of carbonic acid in water, the conversion of those accumulations of bones and dung into basic and neutral phosphate of lime may readily be accounted for.
It is on a large scale the conversion of bones into apatite, a fact already noticed by Girardin and Preissner, who state that bones buried in the earth under the influence of putrefaction are converted into basic and neutral phosphate of lime, which crystallizes upon the surface and in the interior of the bones in small prisms identical with apatite.
Up to the present time no crystals of the phosphate of lime have been found at Navassa, perhaps because the diggings have not been carried yet at a depth beyond the action of atmospheric influences, which may have disturbed the conditions favorable to crystallization, or it may be because no crystallization has been possible during the rapidity of its formation in such large masses of animal matter in putrefaction. The same causes may explain why no fossil bones have yet been found in the deposits of phosphates.
During this process, so favorable to the expulsion of the gasifiable matters, the ammonia of the dung has been evaporated or washed away, and the phosphatic parts only of the animal excrements left to increase the deposit.
The stalactites of phosphate of lime found at the Island of Ascension in contact with guano, illustrate this transformation.
This formation still going on under our eyes does not require any supposition to explain the simplicity of its process, and would not be contradicted even if, in deeper diggings, it was found, as on the Sombrero Island, that extinct animals, such as land turtles, have been contributors to this vast mass of organic detritus. The globular forms of the grains of phosphates forming the great bulk of this deposit may also be accounted for by the regularity with which the undisturbed decomposition takes place.
Securely nestled in the large cavities of the coral rock against the action of the winds or other causes of disturbance, the spherical form of the grains is the result of the quietness of those recesses.
The largest part of the deposits consists of those loose spherical grains varying from the size of a mustard seed to a buckshot.
​Like in the formation of many sand stones, those grains agglomerate in lumps cemented together by water carrying in solution carbonate of lime, and by the continuous action of the heat of the climate, become mineralized, undergoing a true metamorphism. In places favorably situated the whole mass has become indurated, the oolitic rock thus formed having sometimes the hardness of Feldspar.
This process of cementation was beautifully demonstrated on several heaps of fine phosphate left for some years standing at a place called Brownstone, from where the first shipments were made from the Island. There I found in many places a hard crust one or two inches thick, presenting already all the appearance and nearly the hardness of the phosphatic rock, the analysis showing that carbonate of lime was the cementing substance.
This mode of formation is also exemplified by the fact that the sides of nearly all the cavities, from which the phosphates have been extracted, are coated with a crust of the hardest phosphate, evidently cemented there more rapidly by the water washing the carbonate of lime, and by the heat more intense on this white rock.
In the process of cementation the carbonate of lime dissolved by the rain water containing carbonic acid permeates through the globuliform grains of the phosphate, and by the action of heat, the water being evaporated, leaves the cementing substance to aggregate those grains and run them into rock, subsequently hardened by the action of the constant elevated temperature of the tropics; a double process of mineralization and metamorphism of recent masses of organic substances. The presence of iron and alumina in those deposits as phosphates, and of the oxide of iron as coloring substance, may easily be accounted for.
It is known that in the inorganic structures of marine animals there is some iron. Herberges found, in Spongia Ursa, 6 to 8 per cent. of protoxide, and in the red corals, about one per cent. of peroxide is present. Although made principally of carbonate of lime, the corals contain a small proportion, besides other substances, of alumina and oxide of iron, which must be found also in the limestone rock made by corals.