Water 0.80 Phosphate of lime 93.30 Carbonate of lime 0.50 Chlorine, etc. traces Sand 4.70 ----- 99.30 Several other varieties of mineral phosphates have been imported under the name of guano. The most important is Sombrero Island guano, which is found on a small island in the Gulf of Mexico, where it occurs in a layer said to be forty feet thick. It contains-- Water 8.96 Phosphate of lime 37.71 Phosphates of alumina and iron 44.21 Phosphate of magnesia 4.20 Sulphate of lime 0.86 Carbonate of lime 3.36 Sand 0.70 ------ 100.00 A somewhat similar substance, but in hard crusts, has been imported, under the names of Maracaybo guano, Pyroguanite, etc., which contains-- Water 1.03 Organic matter 6.78 Phosphates 75.69 Alkaline salts 4.91 Sand 11.64 ------ 100.00 Phosphoric acid in the alkaline } 0.78 salts = 1.68 phosphate of lime } These substances are all excellent sources of phosphates, but they are so hard that the plants cannot extract phosphoric acid from them, and they are only useful when made soluble by chemical processes.