20 ARCTIC BASALT PLATEAU 
— perfect and / (100) — less perfect, and by the striation / (001). 
The deviation attains until 8”.3 on (010) and 4°—5° on (100) [c:8], the 
mineral being sometimes triclinic, but nearly and mostly orthorhombic. 
Microchemical tests detected abundant lime, but also soda. 
The optical properties indicate a mineral of the thomsonite group, 
the birefraction being too low and the refractive indexes too high 
as compared with the dates of Michel-Lévy and Lacroix,! the axial 
angle corresponding better to the values of Des Cloizeaux,? found 
on thomsonite from various localities. A series of controlling de- 
terminations of the birefraction of thomsonite made by Césaro? on 
thomsonite from nearly the same localities as that served to Des Cloi- 
zeaux gave lower values lying between 0.0026 and 0.0042 for B—e 
a 0.008 „ Ooi „ vB 
2 0.0138 „ Oo , y=e, 
and of the axial angle 2Ey—=82°—85°, and agrees better with the 
terms above. Prior? states that the double refraction of thomsonite 
sinks below the double refraction of stilbite (desmin), whose bire- 
fraction as given by Michel-Lévy and Lacroix® attains 0.006 (7 — 2) 
on columnar sections, which sections on thomsonite give the bire- 
fraction varying between s—a and y—ß, as given above; these terms 
rise to 0.oo6 (d—«) and 0.022 (y—Pß), conformable to the French authors. 
These discrepancies cannot be explained only by varying content 
of soda and lime in the specimens examined optically, because the 
analyses corresponding to the thomsonites from localities examined 
by Césaro show only small variations in these constituents, the 
raised refractive indexes perhaps being a test of heightened lime 
content, but the lowered double refraction probably connecting with 
the physical conditions during or after the mineral growth; the in- 
1 A. Michel-Lévy et A. Lacroix, Les minéraux des roches, p. 320. 
2 Cited after Dana’s System, 6th ed., p. 607. 
3 G. Césaro, Bulletin d. 1. Classe des Sciences de l’Acad. Roy. de Belgique, 
1908, pp. 259— 257. 
4 G. T. Prior, On Sphaerostilbite. Mineralogical Magazine /2. 1900. p. 27. 
5L.c.p. 319. 
