optical Characters of Jacksonitc. — / \ 1/ic/u 11. 2 5 1 
Silica 46 . 12 
Alumina and a little Fe^ O3 25.91 
Lime 27 . 03 
Soda .85 
No water is reported in this analysis, although the state- 
ment is made that after drying at 100'' cent, it showed less than 
i-io p. c. of water. 
It is from this analysis that this mineral has been consider- 
ed "sans eau," but it is quite probable that at the heat of igni- 
tion its water was driven off. Later determinations by Jackson 
an(J Brush, assigned from 4 to 5 per cent of water to jacksonite. 
Two slides of jacksonite have been examined. Neither of 
them shows a finely spherulitic structure, as such spherules 
are quite rare, but a close, confused and short-fibre structure. 
The fibres are rarely parallel, showing but little elongation. 
They are always of the same sign (positive) thus indicating 
that the axial plane is parallel with the fibration. The double 
refraction is very nearly the same as that of lintonite. Lideed, 
the aspect of the mineral, as viewed under the microscope, is 
very similar to that of lintonite, the only difference being in 
the sign of the bisectrix with respect to the fibration. The 
extinction angle is very small, and sometimes is 0°, but owing 
to the fineness and the overlapping of the fibres it is impossible 
to state its maximum in degrees. The mineral, therefore, 
seems to be monoclinic with the axis n^ nearly parallel with 
the fibres. This pink variety has lower sp. gr. than the green, 
viz. 2.68. 
This mineral, therefore, has optic characters quite different 
from those of prehnite, mesolite and thomsonite, and but 
slightly different from those of lintonite. There is need of 
further chemical analyses of both these substances. If they 
should prove to be of the same species, lintonite. being later, 
sliould give wav to iacksonite. 
