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

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VII.—Extent of the Deposits.
In my description of the Island I have remarked that no signs could be found of violent disturbances during its periods of subsidence and elevation. On the shore platform or on the summit of the Island the limestone has the same appearance, and its cavities present the same fantastic irregularities. Everywhere where it is possible to observe it, the stratification has the same regular horizontality. In sailing around the Island, the lowest beds above the level of the sea present them-selves as a compact rock made of carbonate of lime, with, I would have said, large veins of solid phosphate, if by using the word vein I did not fear to convey a wrong idea of this intermixture of the two rocks. The inferior strata of the Island being evidently the oldest, the intermixture of the two ​varieties of rock—the carbonate and phosphate of lime—shows that the formation of the phosphates has been going on since the Island was first elevated above the level of the sea, or at least that the cavities in the limestone have been filled to the lowest stratum, and probably below the level of the sea, to the depth where the coral zoophytes commenced their structure.
Besides the evidences before our eyes at Navassa, this supposition is justified by analogy with the formation of the Island of Sombrero, where they dig now by blasting under the level of the sea, a phosphate bearing a close resemblance to the Navassa phosphate. The difference with the Island of Sombrero is, that being of less elevation than Navassa, no arenaceous phosphate is found there, as it must have been washed out by the sea. The same thing exists on the low beach at Navassa, where by the washing of the waves, only the hard phosphate is left.
Of the early formation of the phosphate we have also abundant proof in breaking the limestone in the diggings of the shore platform, a relatively ancient part of the Island, where phosphate of lime, and sometimes blue specks of phosphate of iron, are found imbedded in the hard limestone, as I have stated before.
From the homogeneous character of the rock from the top to the bottom of the Island, and the facts to which I have just alluded, we may safely conclude that the depth of the deposits reaches the lowest part of the coral limestone, and the lowest parts being probably the hardest, the richness of the phosphate will increase with the depth.
Up to the present time, the phosphates shipped from Navassa have been dug out only from a few acres on the summit of the Island and on the shore platform, without blasting. The deepest cavities from which the phosphates have been extracted seldom exceed twenty feet, the whole average depth being not more than ten feet. During my presence on the Island, I gave great attention to discover some system or regularity in the disposition of the cavities containing the phosphates, but was every day more convinced of their absolute want of system.
With that conclusion before me, the deposits to be worked ​out by the actual mode of labor being extensive enough to last a great number of years, I could not advise to resort to blasting as a systematic way of working, but to use blasts only when it would be found useful to increase the size of a cavity to give easier room to the laborer in digging out the phosphate. By doing so, in many places the deposits will be followed to a great depth with more advantage, without much increase of expense, and the field of operations will eventually be enlarged by deeper workings.
From one acre of land, not quite worked out, at an average depth of about ten feet, during the time we spent on the Island, about 4,000 tons of phosphates were shipped. As I have remarked, all the cavities of the coral rock are filled with the deposits, and where the phosphates are not visible at the surface, it is because they are covered up by the piles of stones of the reefs, or the loose rocks that have tumbled down from the cliffs.
These facts and what I have just said of the depth of the deposits, will leave no doubt of the existence at Navassa of an almost inexhaustible deposit of the richest Phosphatic Guano.