In a letter to “Nature” in 1937, Mitchell and Muir directed attention to the existence, in certain Scottish soils derived from basic igneous parent material, of a very high total cation exchange capacity relative to the clay content—as high as 9 m. eq./gm. clay in some cases. These values were so high as effectively to preclude the possibility of the clay itself being solely responsible for the cation exchange capacity, and in fact in those cases where it was very high relative to clay content, the clay content itself was generally low, so that the cation exchange capacity of the whole soil was quite moderate, of the order of 0·3 m. eq./gm. The high cation exchange capacity could not be due to organic material, for it often occurred in lower layers where the organic matter content was likely to be low.