The quantitative estimation of minerals in rock sections is generally recommended as a valuable exercise, but in practice it is far too seldom performed. The reason is that the recognized methods of estimation are very tedious, while the results, when obtained, have not hitherto been put to any systematic use in the classification of rocks. The usual methods are of two kinds, viz., (I) separations, either gravitational or magnetic, the separated portions being weighed directly; and (2) geometrical methods, involving measurement either of areas or of diameters, and sub-