Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/386

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Mr. William Phillips on the Oxyd of Tin.

234, by which it will be seen that the triangular faces correspond with those of 1, 1, fig. 232, which is that of a common prismatic crystal, fig. 27, Pl. 16, placed in such a point of view, as most easily shews the section shewn on fig. 194, Pl. 24. Each of the eight solid angles of this figure, therefore, is composed of two portions of the common prism, fig. 196; giving, by the common goniometer, along the edges a b c, an angle somewhat more than 112°, which, generally speaking, is the same as that l m f; fig. 200; the plane 1 on 1, over the edge between them, which is that of the common prism, also gives an incidence of 90° corresponding with that of the planes 1 on 1, fig. 196. That part of fig. 234, comprehended within the dotted lines, is supplied in the macle itself, by an elongation proceeding from the upper and lower triangular planes.

Fig. 235, Pl. 26, represents a macle, in which two halves of one similar to fig. 218, are attached so as to give an incidence of the planes P on P, which are not visible in the drawing, but which are parallel with the planes P P, which are given, of 112°, 10′, by the reflecting goniometer, over the angle between them, corresponding with that of the planes 4 on 4 on the summits of the same figure, and of course with that of the planes 4 on 4 on the summits of fig. 210.