On Hyalite. 
241 
the sections were made — had caused to appear within the material any 
structures which might be attributable to a release from internal strains 
consequent on the destruction of the spheroid. Nothing of this nature was 
observed. Occasionally a section would break along irregular cracks, 
but, generally speaking, they retained their circular outline unbroken and 
did not show, when examined under the microscope in ordinary transmitted 
light, any signs of a strained internal concentric structure. 
The Colourless and Water-clear Hyalite. 
Diametral Sections. 
(a) Examination in plane polarised light. 
A diametral section of this variety, when placed on the stage of the 
microscope so that the axis of the microscope passes through the centre 
of the spheroid, exhibits, when examined in plane polarised light between 
crossed nicols, a figure consisting of a black cross with isochromatic rings 
resembling that given by a uniaxial mineral in convergent polarised light. 
On the insertion of a quarter undulation mica plate the figure behaves in 
a similar manner to that of a negative uniaxial substance ; many sections 
were tested in this way, and in all cases the sign was negative. A move- 
ment of the section, however, causes a movement of the figure, so that 
whenever the section is excentrically placed the figure resembles and behaves 
like a partial uniaxial figure. On this account we*do not regard this figure 
given by hyalite as being analogous with that of a uniaxial mineral in con- 
vergent polarised light ; sections of radiating chalcedony (described by 
Miers as a biaxial mineral) and diametral sections of oolitic grains have 
been observed to give, under similar conditions, figures resembling, both 
in general appearance and optical behaviour, those given by hyalite. 
(b) Examination in convergent polarised light. 
The behaviour of diametral sections in convergent polarised light is 
essentially similar to their behaviour in plane polarised light. 
Tangential Sections. 
(a) Examination in plane polarised light. 
Tangential sections, between crossed nicols, are either quite isotropic 
or show very slight graying marginally. 
(b) Examination in convergent polarised light. 
In convergent light a tangential section yields a figure composed solely 
of a black cross ; owing to the thinness of the section and the weak strength 
