Mesolite from Mitmesota. — Wmchell. 229 
double refraction. These contrasts are sometimes strongly 
brought out. For instance: in a section cut about parallel 
with a lot of fibres the general obscurity of the field may be 
relieved by scattered rhombs of very light and bright aspect, 
whose perfect forms are bounded b)' right lines and which 
occur sometimes in regular succession across the field of the 
microscope. Such rhombs are sometimes nearly square, but 
may also become much elongated in the direction of their 
greater axes. As they become longer they also become 
darker, and finally by continuing their elongation they blend 
into the general structure and disappear in the fibrous mass 
in which they lie. In other words, the fibres that happen to 
be cut obliquely, or perpendicularly, give more light than 
those that are cut parallel. This only shows that the plane 
of the optic axes is perpedicular to the elongation of the 
fibres. The same fact can be shown by the use of a section 
cut parallel to the fibres, by the application of the quartz of 
sensitive tint, when it will be seen that some are red and 
some are blue, although lying adjacent, i. e. some have ji^ 
perpendicular and a negative elongation, and others have n^, 
perpendicular with a positive elongation. 
Chemically, mesolite is quite similar to thomsonite, being 
a silicate of alumina, lime and soda, with about 12 per cent, 
of water. If the specimen be somewhat altered by weather- 
ing, and if at the sam.e time thomsonite and mesolite be in- 
timately ingrown, as sometimes happens, it is evident that 
the result of a chemical analysis would be a poor basis on 
which to name the specimen. 
The optic angle in mesolite is large, and in thomsonite it 
is small. This difference introduces another evident optical 
character by which the two minerals can be distinguished: 
viz. sections of mesolite cut parallel to the fibres are almost 
uniformly illuminated whether they present n^ or n^ perpen- 
dicularly, the amount of light along those axes being about 
the same; but a section or thomsonite cut in the same way 
will exhibit certain fibres that are much lighter than the 
others. Thus quite light and quite dark fibres may alternate, 
or they ma)' occur in bundles. The light fibres are cut per- 
pendicular to Wj, in the greater optic angle and have positive 
elongation, while the dark fibres are cut perpendicular to it^, 
in the optic angle and have negative elongation. 
