Thomsonite and Lintonite from Lake Superior. — Wmckell. 347 
[Coatributions to the Mineralogy of Mianesota. II.] 
THOMSONITE AND LINTONITE FROM THE NORTH 
SHORE OF LAKE SUPERIOR. 
By N. H. WiNCHELL,, Minneapolis, Minn. 
Thomsofiite. The megascopic characters of this mineral, 
as already stated, are somewhat different from those of meso- 
lite. It is not known to possess those peculiar cat's-eye mark- 
ings; its fibres are coarse, and the cavities in which it is found 
are large, and of irregular branching shapes, as compared with 
the round or oval masses of mesolite. It is white and diver- 
gently fibrous, and sometimes it is found in the same masses 
with mesolite, with which it is intimately interlocked. It 
seems to take some part in the recurring bands of color which 
constitute the marked characteristic of mesolite. 
Its microscopic characters differ from those of mesolite 
not only in the manner already mentioned under mesolite, but 
in the spreading and ofter fern-like forms which the fibres 
assume, as they radiate and develop from a common point. 
This mineral is orthorhombic, and has strictly parallel ex- 
tinction. Its optic plane is parallel to the base 001, and hence 
its fibres show sometimes a positive and sometimes a negative 
elongation. Its optic angle, which contains Jig. is small, and 
hence sections perpendicular to //g are much darker than 
those perpendicular to iip. The double refraction is strong 
(0.027, Lacioix). 
It has already been stated that this zeolite is sometimes 
intimately mingled with mesolite. This but rarely occurs as 
straight, conspicuous, independent fibres of mesolite, piercing 
obliquely the thomsonite mass, but has been seen rather to 
consist in close, nearly parallel, minute fibres which appear 
at certain zones in the recurring colored bands of the meso- 
lite. They are distinguished then by their greater light when 
they happen to present the axis n^ perpendicular. The axis n^ 
is so dark that it cannot be distinguished in the fine mesh, from 
the general mesolite mass, but those that happen to be cut 
perpendicular to ;/p appear as bright, short needles, sharp at 
both ends, which are quite light compared with the rest of the 
section. This is seen in sections of the zeolites from Terrace 
point, (535 A). 
