MICROSCOPIC STRUCTURE OF FELSPARS. 433 



angle of 115° with one another, as roughly represented in fig. 2, 

 PI. XXIV., their stems (the primary processes) deviating from the 

 original main stem by about fifty-seven and a half degrees. These 

 must, however, be regarded only as roughly approximate measure- 

 ments. 



In fig. 5, Plate XXIV., another crystal from the same section is 

 represented. In this instance there is a midrib which bisects the 

 crystal longitudinally for some distance. It then bifurcates, the 

 forks passing to the angles of the crystal. The angle between this 

 fork is approximately 42° ; I do not think it follows that because of 

 this great discrepancy in the angles formed by the structural lines 

 in these crystals they are therefore different minerals. 



On measuring roughly the angles formed by a drop of chloride-of- 

 ammonium solution evaporated on a glass slip, I noticed very great 

 variation in the angles formed, some of them ranging between 

 72° and 90°. Pine striae follow the directions of these forked 

 boundaries in every part of the crystal, intersecting one another at 

 the same angle, viz. 42°. The stria? in these crystals from the ob- 

 sidian differ from those so characteristic of the Norwegian, Swedish, 

 and other massive examples of orthoclase, inasmuch as they are 

 distinctly visible by ordinary illumination, and under crossed Nicols 

 merely appear white, while only the faintest indications of the other 

 striae are occasionally visible under such circumstances, and polarize, 

 at times, in the most brilliant colours. There seems therefore but 

 little analogy between them. Still it is noteworthy that the great 

 divisional markings in these crystals bear in some instances a strong- 

 resemblance to the great divisional markings in the felspar crystals 

 of the Berkum trachyte already alluded to ; and although I cannot 

 say precisely to what mineral species the crystals in the Mexican 

 obsidian may be referred, still it seems quite possible that they are 

 felspars of some kind ; and their striking resemblance to the little 

 felspar crystals in the Berkum trachyte, in the arrangement of the 

 larger divisional markings, sufficiently warrants me, I think, in en- 

 deavouring to draw some comparison between them. In fig. 6, 

 Plate XXIV., portions of some plagioclase crystals from the gabbro of 

 Volpersdorf, near Neurode, in Silesia, are represented. The right- 

 hand crystal shows several well-marked transverse bands, which 

 cross the longitudinal ones at right angles. The left-hand crystal 

 exhibits only the usual longitudinal bands. I have no doubt that 

 both of these crystals are plagioclase ; and it is therefore interesting 

 to find an example of a triclinic felspar in which cross-banded 

 structure is developed, no matter on how trivial a scale. Pigures 4 

 and 9, PI. XXIV. show similar appearances in crystals imbedded in 

 basalt from Cleveland, I have now directed attention, so far as I 

 am able, to instances in which we find orthoclase displaying com- 

 plete cross-hatching, partial cross-hatching and failure of striation, 

 partial cross-hatching and simple longitudinal banding and, as in 

 fig. 3, PL XXIV., apparently partial transverse striation coupled 

 with an absence of striae. I have also pointed out the existence of 

 peculiar divisional markings which sometimes divide cross-hatched 



