Gabbroid Rocks of Minnesota. — Winch ell. 275 
hardness is about 5. The Histre is vitreous and bright when 
well crystallized. The birefringence is about .006. The mineral 
is biaxial and positive with a large optic angle. The disper- 
sion is not noticeable. The elongation is positive or negative, 
since the optic plane is perpendicular to the elongation. Fibers 
lying in the plane of the thin section seem nearly monorefrin- 
gent. The maximum extinction angle in sections parallel to 
the vertical axis is about 5° in the preparations studied. 
The mineral is often rendered less transparent by a cloud- 
like, amorphous, grayish-white decomposition product. Mes- 
olite is soluble in the acids leaving a residue of gelatinous si- 
hca. Microchemical reactions show the presence of alumina, 
calcium and sodium, the last relatively in small amount. 
Scolccitc whose derivation from the labradorite is just as 
clear as that of the mesolite is always fibrous, elongated par- 
allel to the vertical axis. The fibers are usually radially 
grouped, or nearly parallel. The refringence is inferior to that 
of Canada balsam while the birefringence is markedly super- 
ior to that of labradorite and seems to be as high as .012. The 
extinction angle in sections parallel to the elongation exceeds 
12°. 
Scolecite often includes minute inclusions, of secondary 
origin like itself, of hematite, chlorites, etc. It is much less 
abundant at Carlton peak than mesolite. 
Pseudomesolitc is a new zeolite which is very similar, in 
chemical composition to mesolite, from which it differs, how- 
ever, very distinctly in optical characters. Hence the name by 
which it is proposed to designate the mineral. 
From its optical characters the mineral is referred to the 
monoclinic or more probably the triclinic system. Pseudomes- 
olite is eminently fibrous, in very fine fibers radially ar- 
ranged or occasionally interlaced. In the spherulitic masses 
microscopic short fibers occur rather uncommonly making 
various, but usually small, angles with the general direction of 
the other fibers. These radial aggregations are both micro- 
scopic and megascopic in size, and it was from a group of 
the latter that material was obtained for careful study and 
chemical analysis. Twinning is rare, indeed it has not yet 
been observed; parallel fibers are usually oriented alike. Pseu- 
domesolitc has two good cleavages very nearly at right angles, 
