Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/377

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367
Mr. William Phillips on the Oxyd of Tin.

by that of hemitrope. The seeming confusion produced by the macle,[1] is very often much augmented by circumstances apparently resulting from no law, by which parts of crystals are jumbled together, so as to form a whole, that can only be understood by a long and patient investigation, which in the end serves only to satisfy the observer of the absence of all regularity in the disposition of the several constituent parts, although each may be separately defined.

But even the regular macles of the oxyd of tin seem, at first sight, to form no very intelligible part of the series of its crystallisation, although they are in fact very interesting. To understand them it needs only to become well acquainted with some of the most simple; as, for instance, with those of figs. 186, 187, 188, and 189, Pl. 24, which will serve as a ready clue to the comprehending of all such as are of regular formation; and by these it will be seen that they all proceed from the same law of section.

The macle first described by De Lille, who ascribes to Lhermina the development of the law by which it takes place, is that of fig. 186, and will be understood by referring to fig. 190, on which the dotted lines a b c d e represent a section of it, parallel with the edges e f of the upper and g h of the lower pyramid[2] dividing the crystal into two parts. The upper part is represented in the same

  1. I have retained the term macle in preference to that of hemitrope, because the latter does not in fact apply to any one of them. It does not seem to me that the term macle is objectionable, because it has been given to a substance. In this case it only denotes a circumstance, and no one would think of asking for macles, without adding, of tin, of the ruby, &c.
  2. The section by which this, as well as the succeeding macles, takes place, being parallel with the edge of the secondary pyramid, it follows of course, as a reference to the series of the second modification will shew, that this section must also be parallel with the faces of the primitive octohedron.