XXXIX. The Crystallisation of Rocks

Abstract

To a greater degree than most branches of petrography the study of the crystallisation of rocks involves principles that are closely akin to those of physical geology. No geologist, therefore, hardly even the most rigorous specialist in palaeontology, can afford to neglect the modern views regarding the origin of igneous rocks. These rocks have all originated from reservoirs of fused material containing as important constituents various dissolved gases. Recent work has shown that this molten magma is best looked on as a mixed solution and applicable to exactly the same treatment as an aqueous solution—a view first put forward by Bunsen forty years ago. Diffusion, osmotic pressure, and ionisation must all be considered. We may even refer to minerals, such as the plagioclase felspar series, as solid solutions. This magma is not a mixture of any substances whatsoever in any ratio, but certain substances are only present within certain limits, just as salts are in a solution. A magma containing a high percentage of silica always contains in addition a comparatively large amount of alkalies and a small amount of lime, magnesia, or iron oxide. This method of treating the magma as a solution is very helpful in explaining the perplexing order of crystallisation of the minerals of rocks. Quartz, which is very infusible, often crystallises last. But with watery solutions we know that the temperature of crystallisation varies with the amount of substance dissolved, and the same principle makes clear that the order of super-saturation, and thus of crystallisation This 250-word extract was created in the absence of an abstract