Of the large class of " rock-" or " crust-" guanos from the coral islands of the Caribbean Sea many are called "guanos," without any regard to their origin, which in some cases remains in obscurity. Seeing that they contain no trace of nitrogen, the term " phosphate " seems more appropriate.
The most important and valuable variety is named after the island of Sombrero, on which it is found. The islet, which is only about 2f miles long, mile wide, and 20-30 ft. above the level of the sea, may almost be said to be composed of phosphatic materials ; and the fragments of bones, found in the rock, have led to the supposition that the remains of turtles and other marine animals may have collected in the coral, while it was yet a shoal, and that bird-droppings assisted in cementing the mass together. The phosphate varies in colour, and is sometimes porous, at other times dense. It is at present worked below the waves, and probably the coral foundation on which it rests is now almost reached, as the mineral contains much more carbonate of lime, with less of iron and alumina, than formerly. Dissolved alone, makes a very superior superphosphate, of light-yellow hue. It contains :—Tribasic phosphate of lime, 69-76 per cent. ; carbonate of lime, 12-17 ; iron and alumina, 4-10 ; insoluble matters, 1-2.
Navasaa phosphate exists in the form of pisolitic grains, of bright-red colour, cemented into hard masses, in the rock-cavities of the island whence it is named. It contains only a moderate amount of carbonate of lime, but its proportion of iron, and still more of alumina, is so great that it is impossible to make a superior superphosphate from it alone ; moreover, the toughness and stickiness of the material during manufacture, and the hardness with which it ultimately sets, are additional drawbacks. Its large proportion of phosphate makes it useful for admixture with poorer materials. Its principal component parts are :—Tribasio phosphate of lime, 55-70 per cent.; carbonate of lime, 4-6 ; fluoride of lime, 0-2 ; iron and alumina, 23-28 ; insoluble matters, 31-5.
Maracaibo or Monk's Island produced a very superior phosphate, which was principally used in manufacturing the so-called " phospho-guano," but which is now exhausted, or nearly so.
St. Martin's Island, of the same group, now yields a valuable article, but which sometimes contains a large proportion of carbonate of lime. It varies thus :—Tribasic phosphate of lime, 524
764 per cent. ; carbonate of lime, 15-324 ; iron and alumina, 21-44.
The island of Curacao furnishes a valuable phosphate of lime, in an unmineralized and finely divided state, which may be applied in its natural state, or employed to manufacture very superior superphosphates, as much as 38 per cent. of soluble tribasic phosphates having been got out of a damaged sample showing only 65 per cent. of tribasic phosphate before treatment. The possibility of attaining such a high standard, even from a comparatively inferior sample, is due to the fact that none of the phosphate appears to be contaminated to any extent with iron and alumina, and that carbonate of lime and siliceous matters are almost equally conspicuous by their absence. The proportion of tribasic phosphate of lime rises as high as 80 per cent., and averages about 70.
Another of the Leeward Islands, called Oruba, yields a tolerably rich phosphate, but it is apt to be strongly contaminated with iron and alumina. It chief ingredients are :—Tribasic phosphate of lime, 631-764 per cent.; carbonate of lime, 24-151 ; iron and alumina, 144-26f.
Redonda and Alta Vela Islands produce phosphates in which the lime has been entirely, or nearly so, replaced by alumina ; these are, therefore, quite valueless for manuring purposes.
There remain but the phosphatic guanos to be mentioned. The best of these is procured from Mejillones, on the Bolivian coast, and is employed in manufacturing the compound known as " biphosphated guano." Besides about 71 per cent. of trihasic phosphate of lime, it has nearly 1 per cent. of nitrogen, with less than 2 per cent. each of carbonate of lime and siliceous matters, and scarcely any iron and alumina.
Browse Island guano is closely similar to Mejillones, but somewhat superior. It is imported in a fine powder, free from lumps and stones, and ready for treatment with sulphuric acid, without any grinding or other preparation. It contains about 1i per cent. of ammonia, and yields a superphos phate up to 40 per cent. soluble ; at the same time, it contains no fluorine, very little iron and alumina, and only a convenient proportion of carbonate of lime.