Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/385

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Mr. William Phillips on the Oxyd of Tin.

Pl. 26, and as fig. 222 is composed of four equal parts of macles similar to fig. 223, it will follow that the lines a, b. and b, c, fig. 224, will represent that portion of the whole plane, occupied by the constituent part of one macle, and further, the lines of section, a b and b c, being perpendicular to the lines a d and c d, that the section of each of the four macles constituting that described by fig. 222, takes place parallel with the planes of the common prism; and it has been shewn, in treating of the primitive crystal, that in this direction a cleavage is easily obtained.

Those described by figs. 225 and 226 differ only from fig. 222, in this, that the planes of some other modifications are visible, the respective numbers of which are placed on them.

That described by fig 227, consists of four macles similar to that delineated by fig. 211, except that in this, each is elongated in the direction shewn by fig. 228. Fig 229 represents one composed of four elongated macles fig. 214.

By fig. 230, Pl. 26, is represented a singular combination of the four macles composing the preceding figure, placed on the edges of the prism of a crystal similar to fig. 42, pl. 17. This combination may be quoted in evidence to the truth of what has been said of the construction of common macles, for it will be observed that the faces 1, 1, on the prism of the crystal itself, and on the macles placed on its edges, are all planes of the first modification. Both terminations of fig. 230, as well as of the two preceding figures are visible on the macles.

Fig. 231, Pl. 26. represents a macle composed of 16 portions of the prism, fig. 196, Pl. 24, each elongated, the whole forming an octangular prism, of which a horizontal section is described by fig. 233. The striæ are uniformly in the direction represented. The construction of this macle will be obvious by consulting fig.