This article provides an example of the application of the method of depicting the chemical compositions of rocks using vectors to the study of the phenomena of magma differentiation and magmatic evolution. As such an example, I took the only one of its kind, the most well-studied case, namely the famous volcano Vesuvius in the history of mankind. In the depths of igneous bodies - plutons - processes occur under conditions significantly different from those in which we directly observe magma reaching the earth's surface in the form of lava. Compared to other volcanoes, Vesuvius, thanks to its study, has the advantage that here we even have some data on the progress of the process over time. Regardless of this or that hypothesis, the presence of a shift in the line of differentiation of lavas with each new cycle of volcanic activity can be considered established for the chemistry of rocks. This displacement turns the line of differentiation into a strip of points or vectors. This arrangement of elements, depicting the chemical compositions of rocks in the form of a strip, is a characteristic feature of almost all such diagrams. The general arrangement of these geometric elements, the position of some of the axial lines of their stripes provide a clear general characteristic of the chemistry of the volcanic and plutonic formations presented in the diagram. But this is only the first step towards studying their chemistry with the help of a diagram.
