IN regard to statements in NATURE of June 23 and July 21, 1921 (vol. 107, pp. 523 and 652) to the effect that the curious ridges and furrows which occur in mortar in walls are due to the segregation of lime, I would invite attention to a note in Proc. Dorset Nat. Hist, and Antiq. Field Club, 1906, vol. xxvii. p. xxxii, giving an account of an exhibit of mine of a series of pieces of mortar from a wall showing the early stages of the development of this phenomenon. The appearance is caused by the growth of moss in minute shrinkage cracks in the mortar, the sides of the cracks being gradually disintegrated by the roots of the moss, until the final stage of ridge and furrow is reached and the moss, not having sufficient root-hold, falls out when dry.