From time to time I have received from correspondents suggestions that the method of determining the geological age of the Earth by the rate of solvent denudation of sodium might be open to considerable error if the allowance made in my paper (Trans. R.D.S., ser. ii, vol. vii), for sodium chloride carried from the sea by winds and washed from the atmosphere by rain, was seriously at fault. These suggestions arise from incomplete study of the quantities involved. Had more space been given in my paper to this question, the hasty criticisms I have had to contend with, doubtless, would be less often advanced. The whole matter is capable of the simplest arithmetical statement, and the limit of error arising from this source easily defined. Recently one gentleman has written at considerable length on the matter in the pages of the Chemical News. I have replied to Mr. Ackroyd in that journal. But the definition of the limit of error referred to, and the consideration of some other points raised in the discussion, are more in place in a geological than in a chemical journal. I would therefore seek for space in the Geological Magazine wherein to repeat in part what I have said in the Chemical News, adding some matters more especially suited to geological readers.