Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/379

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Mr. William Phillips on the Oxyd of Tin.

angle, described by the dotted lines, must exist between the planes 2, 2, of each, which are those of the second modification; instead of which the whole space between those planes is occupied by an elongation proceeding from each, so as to connect and form the two upper planes, 2, 2, into one plane; the same effect takes place in regard to the two lower.

The series of this double macle is given by figs. 216 and 221, Pl. 25, placed in the position in which they are most commonly found: they usually present but one termination, the other being imbedded in the matrix. Fig. 217, represents a crystal similar to that of fig. 190, but with a shorter prism, so placed as to shew most advantageously the section described on fig. 190, and thereby serve as a clue to the more ready comprehension of the series. On each figure the planes of the several modifications are pointed out, by the number of the modification itself being placed on them. On figs. 218, 219, and 221, the planes of the primitive form are visible. These macles are generally delined with great neatness, and mostly allow of the perfect use of the reflecting goniometer, which has been employed to corroborate what has been said of their construction, the truth of which it places beyond a doubt. I possess macles represented by figs. 218 and 220, on which both terminations are complete.

Incidence of 4 on 4, on the summit of fig. 218─112°. 10′.

This macle seems to verify the conclusion of Lhermina, that the section a b c d, fig. 190, takes place parallel with the edges e f and g h, which are those of the secondary pyramid. If the terminations k k of fig. 187, were complete, or, in other words, if the planes of the second modification were not visible, fig. 187 would take the form of fig. 192. Of fig. 192, let a section along the

Vol. ii.
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