The Geological Magazine did me the honour in 1914 to publish a paper in which I proposed a classification of eruptive rocks on the basis of the “degree of saturation” of the constituent minerals with silica. Any mineral which is proved by its field associations to be stable in the presence of free silica, under such conditions as prevail in natural rock magmas, was said to be “saturated” with regard to silica. Other minerals, such as leucite and olivine, which are not stable in presence of free silica, within the ordinary range of magmatic temperatures, were said to be “unsaturated” with regard to silica. Eruptive rocks were then classified as “oversaturated”, if they contained quartz or tridymite; “under-saturated”, if they contained any unsaturated minerals; and “saturated” if neither free silica nor any unsaturated mineral was present.