Simple rib shapes of trigonal and hexagonal syngonies. For crystals of trigonal and hexagonal syngonies we have derived 90 simple rib shapes. 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 rib forms.In the derivation of simple rib forms of cubic singony, 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 rib 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 twelve- tsatyrebernik of a cube.

The existing petrographic classification understands tuffites as complex rocks with pyroclastic and subordinate quantitatively 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 are simultaneously accumulated in the water basin, mixing with each other. Naturally, organic remains, important to the geologist and usually of little importance in the construction of the rock, may also be found in the composition of such a rock.