28 Scientific Proceedings, Royal Dublin Society. 
manner, others show distinctly a different orientation of the sex- 
tants. The directions of maximum extinction of light make 
various angles in the different parts of one tablet. The super- 
position of very thin tablets turned one over another about the 
vertical axis, causes some uncertainty in the definition of those 
angles. 
The very thin and partly altered tablets of the Tardree Moun- 
tain specimens are not well suited to the precise investigation of 
their optical properties, but because it was on these crystals that 
I first stated the optical anomalies of tridymite, some further 
investigations on the same subject may find place here. The very 
clear and relatively great tablets of tridymite of the Trachyte of 
the Perlenhardt in the Siebengeberge, near Bonn, on the Rhine, 
gave me very precise results, and prove that the tridymite is not 
hexagonal and uniaxial, but distinctly biaxial. Iam able to render 
the interference figure visible in the polarising microscope with 
all the sharpness that can be desired. One can see the two black 
hyperbolas marking the poles of the optical axis. The axial 
angle must be large as the hyperbolas are far apart. The minute- 
ness of the objects have not yet permitted an exact measurement 
of the axial angle. The plane of the axis seems to be normal to 
the hexagonal base of the tablets; its intersection with it joins 
two alternating edges of the base. The direction of maximum 
extinction bisects the angle of the edges. In some tablets, com- 
bined of different parts, the phenomenon appears almost as 
represented in figure 2, which is diagrammatic. As the plane 
of the optical axis indicates a pinacoidal plane of the crystal and 
the other direction of maximum extinction, the other pinacoidal 
plane, normal to the former, the side of the basal hexagon of 
Tridymite can only be supposed to be formed by the two pris- 
matic sides (110) of two crystals joined with a prismatical plane 
(120).. We have then here a somewhat similar growing together 
of twin crystals as we know of in aragonite, witherite and millerite, 
the latter lately described by Descloizeaux;* all twins of the ortho- 
rhombic system, presenting a pseudo-hexagonal form. A further 
investigation of the crystals of tridymite will prove whether all 
its hexagonal tablets have the same or a similar structure. The 
“ Neues Jahrb. fiir Mineralogie 1877, i. p. 42. 
