VI.—The Principle of Saturation

Abstract

In Professor Shand's reply to my criticism of his original paper on the application of the principle of saturation with respect to silica in the classification of igneous rocks, some points arise which make it clear that he has misunderstood me in several particulars. In the beginning of my paper, it was explicitly stated that what was to be considered was (1) whether the principle, as enunciated by Professor Shand, could be extended to minerals other than the silica ones (quartz, tridymite, etc.), the felspars and the felspathoids, and (2) whether the criterion whereby the rock minerals were divided into the two groups of saturated and unsaturated was sufficiently exact. The criterion used was that of “the observed facts of distribution”, and any mineral which was found coexisting with quartz or some other form of silica in igneous rocks was said to be saturated, while those which did not occur along with quartz were said to be unsaturated. Correspondingly, rocks which consisted entirely of minerals of the former type were classed as saturated, those consisting of mixtures of the two types as part-saturated, and those consisting solely of the second type as unsaturated. I endeavoured to show that the rock-forming minerals cannot be satisfactorily divided into two classes in this empirical fashion, without any consideration of the cooling-histories of the individual rocks in which the minerals may be found.