Simple edge forms of trigonal and hexagonal syngonies. For crystals of trigonal and hexagonal syngonies we have derived 90 simple edge forms. In order to classify them, we will use the numbering and special symbols adopted for tetragonal forms.

The present communication completes the series of articles by V. I. Mikheev and I. I. Shafranovsky devoted to the complete derivation of all possible edge forms. In the derivation of simple edge forms of the cubic system, we first of all proceed from possible facet combinations. For this purpose we had to enumerate all possible pairwise combinations separately for each of the five kinds of symmetry. The symbols for such combinations are summarized in the table, where the upper horizontal line contains the symbols of the faces of the initial forms, in the planes of which lie the derived edges. The vertical line contains combinations of symbols of those faces, the intersection of which results in edge forms. A number of edge forms with a relatively small number of included edges are the result of intersection of faces of one simple form (from among those indicated in the upper horizontal row of the table); for example, the dodecahedron of the cube.

The existing petrographic classification defines tuffites as complex rocks with pyroclastic material and, quantitatively subordinate to it, normal sedimentary material. The content of the latter is most often specified in the range of 10‑15%. Such a peculiar composition of tuffites results from the specific conditions of their formation, when the products of volcanic eruptions and sedimentary, most often clastic material accumulate simultaneously in a water basin, mixing with each other. Naturally, organic remains, important to the geologist and usually of little importance in the rock composition, may also be found in the composition of such a rock.