This paper discusses the conditions for the formation of red sandstones, to some extent from the point of view of field geology, but mainly from that of laboratory experiment. Field observation made in the Tyne valley confirmed a suggestion made to me by Sir Robert Robertson that, in some cases at least, red sandstone may derive its iron from chalybeate water, which is a solution of ferrous bicarbonate. Starting with this idea, it was found by experiment that red or yellow sandstone could be produced according to the circumstances under which the chalybeate water deposited its iron. If the chalybeate water is evaporated on the surface of a solid body, whether a sand grain or the surface of a glass bulb, a red coating results. The same occurs when evaporation occurs in air from an undisturbed liquid surface, a red skin forming on the liquid. If, on the other hand, the liquid is agitated with air, as in a stream in the open, a light yellow precipitate forms throughout it. This when formed settles down as a yellow deposit on sand grains, twigs, or other solid surfaces, or on solid sandstone rocks. The red and the yellow deposits both consist of hydrated ferric oxides. It is difficult to say from a chemical point of view what the essential difference of constitution between them is. The degree of hydration is not rigidly constant for different red preparations, and tends to be somewhat less for the red material than for the yellow. The main point is that red sandstone has been produced which closely imitates the natural product. This has been done without using high temperatures and using only material (chalybeate water) of common occurrence in nature. The natural red grains were mounted side by side with the artificial ones and examined with the microscope using a low power and a dark ground illumination. While both samples showed a considerable range of intensity in the red colour of the individual grains, they were as a whole indistinguishable as to the colour and its mode of distribution. It is believed that dehydration by volcanic heat or other agency, either of the sandstone itself or of its constituents, has nothing to do with the matter.