In the last volume of the Annals of British Geology (1893), just published, Professor Blake gives an abstract of a paper by me on “Variolite from Roundwood,” and incidentally calls attention to the discordance between the formulæ given by General McMahon, Dr. Roth, and myself, to explain the conversion of olivine into serpentine. I hasten at once to correct a slight error which occurs in my equation, but which, fortunately, does not affect the principal result of the argument, as the correct formula was used in making calculations. Amended, my equation is as follows:—