Phosphatic guanos, as already pointed out, are similar in origin to nitrogenous guanos. In their case, however, the nitrogen, alkalies, and soluble phosphates which they originally contained have been almost entirely lost by the decomposition of their organic matter and the action of water.[194] Most of them still contain very small quantities of nitrogen, amounting to a fraction of a per cent. Of these deposits there are very many occurring on islands in different parts of the world. In appearance the guano obtained from them is very different from nitrogenous guano, being much lighter in colour, and of a fine powdery nature. It forms a very rich phosphatic guano, containing in many cases between 70 and 80 per cent of insoluble phosphate of lime. Such guanos are largely used in the manufacture of high-class superphosphates, by treating them with sulphuric acid. Being of an insoluble nature, they are not very suitable for direct application to the soil. Of these phosphatic guanos the following are the chief--those marked in italics being still unexhausted:-- 1. _Baker_, Jarvis, Howland, Starbuck, Flint, _Enderbury_, _Malden_, Lacepede, _Browse_, _Huon_, _Chesterfield_, _Sydney_, _Phoenix_, _Arbrohlos_, _Shark's Bay_, and _Timor_--all found on islands in the Pacific Ocean. 2. _Mejillones_, on the coast of Bolivia. 3. Aves, _Tortola_, _Mona_, and other deposits in the West Indies. 4. _Kuria Muria_ islands, in the Arabian Gulf. For further particulars as to the composition of these different guanos, the reader is referred to the Appendix, Note V., p. 329. _Inequality in Composition._ That guano was a substance of by no means uniform composition was a fact early recognised in the history of the trade. Not only did guano from different deposits show on analysis different percentages of the manurial ingredients, but different samples of guano from the same deposit were often found to differ very considerably from one another. It soon became the custom, therefore, to sell it on chemical analysis, each separate cargo being carefully analysed. But this custom did not wholly obviate the difficulty, as the guano in even one cargo might differ. In the case of the older and richer guanos, there was certainly more uniformity in quality, but they were liable to differ in their percentage of nitrogen.[195] As, however, the deposits became gradually worked out, their lower layers were found more or less largely admixed with stony and earthy matter, and their composition was naturally rendered very variable. This state of matters was unsatisfactory to buyers and sellers, and led to much friction between the two, as it was found wellnigh impossible on the part of the seller to guarantee the composition of his manure. The custom of preparing the material by reducing it to a fine powder before sending it into the market, and the custom, subsequently introduced, of treating it with sulphuric acid, have done away with this difficulty to a large extent.