Thomsoiiite and Lintonite from Lake Superior. — Winchell. 349 
Although on chemical grounds this mineral is referable 
to mesolite, or to thomsonite, it differs from both in optical 
characters. 
Its axial plane is not perpendicular to the elongation. 
Its acute bisectrix is ;/g, and the acute optic angle is 
very small. Its elongation is negative, i. e. parallel with ;/p. 
Its refraction is less than that of thomsonite, and its double 
refraction is nearly that of quartz. Its angle of extinction 
varies from 0° to 19°. and its fibres are short and irregular in 
form, their width being about one third or one fourth their 
length, without rectilinear boundaries. Sp. Gr. 2.372. 
For the purpose of making an approximate measure of 
the double refraction of this mineral a slide was prepared 
on which were ground simultaneously to the same thinness, 
four plates of barite, two of scolecitc and a pebble of lintonite 
(625B) the last being in the center. In the fine massive, almost 
granular lintonite the highest color, being that presented bv 
sections parallel to the axial plane, compared with the color 
given by the barite in similar position, shows that the double 
refraction of lintonite is about 0.017 to 0.018, that of thomson- 
ite being 0.028. Such comparison is easily made by the use of 
the colored "tableau des birefringences" of Michel Levy, con- 
tained in the work Mineraux des Roches. A comparison 
with scolesite cut in the same position gives the same result. 
This mineral has nearly the same optical and physical char- 
acters as jacksonite, but in chemical composition and specific 
gravity these minerals are quite distinct. The position of the 
optical plane parallel to the fibres allies lintonite with jack- 
sonite. 
Localities of Lintonite. It was found first at Terrace 
point. It is found in the same association at Eclipse beach, 
where it forms a casing that surrounds the mesolite masses, 
the mesolite radiations penetrating it. It is also found on the 
north side of Isle Royale. 
