form a framework within the original crystal of mica arranged in 
the three directions (fig. 3) — or not resorbed at all. 
In the former case hardly any inclusions occur, in the latter ease 
the mica contains very many poikilitically included leucite crystals 
and microlites of augite, and thus the mica is in this case the last 
product of crystallisation. The slightly birefringent sections after the 
basal pmacoid are not defined crystallographically; they show in 
convergent light the sharp bisectrix and optical axes diverging with 
an angle which is unusual for biotite; sections after undetermined 
directions show twinning according Tschermak’s law. The plechroism 
is strong and abnormal: 
, C > * > 
pale orange red canary yellow light yellow to colourless. 
from which it is evident that also the cleavage laminae are strongly 
pleochroitic. 
The resorbed crystals show, in these parts which have been pre¬ 
served, the same pleochroism; moreover a continued growth after 
the resorption period frequently occurs and then the nucleus and the 
periphery are orientated in the same way (fig. 3). 
The isolation of pure mica material was impossible on account of 
the large amount of inclusions, so that the chemical composition can 
throw no further light on the abnormal optical properties. 
The chemical composition of the rock appears from the following 
analyses (I) carried out by F. Pisani and the author. For the sake 
of comparison there are added the analyses of a few other rocks 
which are closely allied mineralogically and chemically. 
I* Mica leucitebasalt. Boulder Oeloe Kajan. Borneo’s Ooster- 
Afdeeling Anal. F. Pisani and H. A. Brouwer. 
II. “Minette of Highwood type”. Arrow Peak. Highwood Moun¬ 
tains. Montana. Anal. H. W. Foote cf. L. V. Pirsson Bull. 
N°. 237 U. S. Geol. Survey. 1905, p. 142. (BaO 0.48; 
SrO 0.25). 
III. Missourite. Shonkin Stock. Shonkin Creek. Highwood Moun¬ 
tains. Mont. Anal. E. B. Hurlburt cf. L. V. Pirsson Bult. 
N°. 237 U. S. Geol. Survey. 1905 p. 115. 
IV. Analcime basalt. The Basin. Cripple Creek Distr. Colorado. 
Anal. W. F. Hillebrand cf. Cross Journ. of Geol. Vol. V 
1897 p. 689. (BaO 0.13; SrO 0.12). 
V. Leuciteabsarokite. Ishawooa Canyon. Wyoming. Anal. J. B- 
Whitfield cf. A. Hague. Am. Journ. of Science Vol. XXXVIII 
