Section 15 deals with the very interesting subjects of desert films and crusts. Two features are very noticeable in many desert limestone rocks, the production of a hard crust on the outside and of a powdery soft interior. The outer hard layer is often of different colour from the internal portion and highly polished. Professor Walther points out that such hardened crusts occur in buildings in Central Europe, where he suggests that during the dressing of the stone the fine powder produced has been so driven into the pores that chemical agencies are no longer as active. I ventured a suggestion for the production of such sub-crystalline surfaces where much exposed to the sand-blast in which action such as the above was vaguely present to my mind, but whatever external physical effect such external activities may have, the more important is undoubtedly the concentration of solutions near the surface under the influence of the solar heat.