Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/378

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Mr. William Phillips on the Oxyd of Tin.

direction in fig. 186, and the same proportion of another crystal having been turned half round, and reversed in its direction, is, in that figure, thus attached to it. The incidence of the edge a b on the edge c d, fig. 186, is 112°.10′.[1]

The series of this macle, which is the most simple of all in its combination, is described by figs. 203, 204, 205, and 206, Pl. 25; and as the planes on each are numbered with those of the several modifications to which they respectively belong, they will be readily understood, except perhaps that of fig. 206. This latter, as a reference to fig. 52 will evince, is composed of similar parts of two crystals; but as the section of the two portions of which it consists took place parallel with a face P of each, which do not appear in the macle itself, it follows of course, that this section must be immediately opposed to that of the three preceding figures. The existence of this section will be explained and confirmed in speaking of the formation of the macle described by fig. 188.

Double Macles.

The macle described by fig. 184, Pl. 24, may be termed a double macle, because it is terminated at each end by a macle similar to that described by fig. 191, which resembles that described by fig. 186, except that the planes 1, 1, which are those of the prism, are shorter.

If we were to suppose fig. 187 to consist of two macles similar to fig. 191, simply reversed, it would be obvious that a re-entering

  1. The Abbé Haüy in his Tableau comparatif, has given this incidence, as 112°.16′.44″, but I have been induced to quote it as above, because I have uniformly so obtained it by means of the reflecting goniometer, on macles having the edges a b and c d replaced by the planes of the fourth modification.