Dr. J. S. Hyland—Soda-Microcline, Kilimandscharo. 161 
In 1886 Mr. Miers! published an account of a felspar from the 
same locality, to which his attention had been called by Prof. Bonney, 
and concluded it to be an orthoclase, similar to that described by 
Brégger?in the Norwegian ‘“ Rhombenporphyr.” A supplementary 
note * was issued in 1887 with analyses by Mr. Fletcher. 
Several loose crystals were included in Dr. Meyer’s collection, 
and these, although not quite fresh, favoured a study and deter- 
mination of their characters. 
The felspar is, when fresh, of a pearl-grey colour, with a highly 
vitreous lustre on both cleavage faces, and possesses the usual 
cleavage parallel to P and M, with a maximum length of 29 milli- 
metres. 
The following combinations were observed: _ 
1. OP (001), © B x (010), oo P (116), 2 P w (201) : wo Boo and 
2 P ow well developed. 
2. OP, 2 P, © Bw : oP (Tand 1) predominating: «© BR o is 
mostly very small and often almost disappears. 
OP is sometimes represented by a cleavage face; the edges of 
the faces are mostly rounded off. 
Mr. Miers has recorded these faces in the article already referred 
to. The presence observed by him of a macroscopic twinning 
according to the Carlsbad law with the striking peculiarity, that 
co Pw (100) appears both as plane of composition and of twinning 
I was able to confirm as regards the second combination only, for 
wherever I found the face y (2 P  ) developed, there was no trace— 
optically or erystallographically—of any such macroscopic structure. 
Hence such crystals are in this respect simple individuals. 
Upon careful examination of these twin-crystals a peculiar 
anomaly becomes apparent. The occurrence of the twin-structure 
demands that the two halves of such a crystal be symmetrical as 
regards co Po; one is in fact led to expect at the one end a roof- 
like top, at the other a re-entering angle (einspringenden Winkel). 
The latter can at once be noticed on our felspar, but the roof-like 
top is wanting, its place being taken by one surface, the face OP. 
By striking this surface a portion flies off, and we recognize the 
characteristic form. Further, a section cut parallel to M (BR a ) 
from a crystal twinned after the Carlsbad type ought to be resolved 
under crossed Nicols into two fields optically different. In contradic- 
tion to this, such sections cut from the outermost portions of a 
crystal behave as if they were derived from un-twinned individuals. 
Tf, on the contrary, a section be cut parallel to the same surface, but 
from a point nearer the geometrical centre, one is able to observe the 
ordinary phenomenon and the inclination of the basal cleavages to 
each other can also be determined: this amounts to 42° 15’. That 
this curious anomaly does not owe its origin to any mistake in the 
directions of the sections, I was able to settle by measurement 
and comparison with the crystals from which the sections were 
1 Mineralogical Magazine, 1887, vii. pp. 10-11. 
Die silurischen Etagen 2 and 3, 1882. 
3 Min. Mag. 1887, p. 131. 
DECADE III.—vVOL. VI.—NO. Iv. 11 
