ANYONE who lives in a fairly wooded part of the country must be familiar with the fact that at certain stages of decay fallen branches of trees are often to be observed among the dry forest-litter coloured more or less through their tissue with various shades of green. After an examination of thin sections with the microscope, I am unable to trace this to any saprophytic organism. Chemical analysis, on the other hand, reveals the presence of iron as the base of the green colouring-matter (using fairly strong nitric acid as a solvent), which—so far as the evidence at present goes—seems to be some organic salt of iron, the organic acid being probably furnished by the slow decomposition of the woody tissue. In the hope that some further light may be thrown on the origin of the green-colouring matter of many Tertiary green earths, I would ask the favour of being allowed to solicit references to any foreign literature of the subject with which any of the numerous readers of NATURE may be acquainted.