Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/374

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Mr. William Phillips on the Oxyd of Tin.

exhibit its planes very differently, and on all the crystals represented by these two figures, they are so uneven, or irregularly striated, as to render it wholly impossible to subject them to the reflecting goniometer. This modification is extremely rare. The crystals described by fig. 108. have I believe been brought only from the mine called Huel Fanny, west of Redruth.

Ninth Modification.

By the ninth modification, as well as by the second, each of the four solid angles formed by the meeting of the two pyramids of the primitive form, is replaced by two triangular planes, placed on the edges of the pyramid, and inclining on the axis passing through the solid angles; but in this modification they incline more on that axis than those of the second modification, fig. 110. Pl. 20.

Fig. 111. shews the planes of this modification in a more advanced state; and fig. 112. shews them in combination with the secondary pyramid. They are generally so minute or so considerably striated, as to prevent their incidence either with the planes of the primitive form, or with those of any other modification, from being satisfactorily obtained.

The greater part of the crystals delineated in the series of this modification, of which the planes of the first modification are long, and on which those of the seventh modification are observable, were taken from the surfaces of rounded portions of grauwacke, found in the hollow parts of the vein in Relistian mine. I have not noticed any macles on the specimens from that mine; all the crystals from it are nearly black, and of remarkable brilliancy.