On Tridymite-Quartztrachyte, and on Olivinegabbro. 27 
The axis of elasticity, or the direction of the maximum extinc- 
tion of the light, seen in slices parallel to the plane of symmetry 
(O10; 2 Px) makesan angle of 11° with the edge between (001) 
and (010) (the base and the plane of symmetry). The plane of the 
optical axis is normal to the plane of symmetry, the angle of the 
optical axis seems not to be the same inall crystals. I found it 
to be (2 E= 24° 30’) 
The clinoclase (triclinic felspar) is likewise clear and pellucid. 
In the sections it forms smaller but longer prisms, with a very 
beautiful chromatic striation of its twin lamelle. The angle 
between the direction of maximum extinction of light and the 
limiting line of the lamelle measured in one case 54° on both 
sides. In little pieces obtained by cleavage parallel to the base, 
I found the angle between the edge (001): (010) and the direction 
of maximum extinction=2}°. Therefore, according to the deter- 
mination of the optical properties of the different varieties of 
felspar by Descloizeaux,* the clinoclase in the present instance 
is probably a var iety of andesine. 
The tridymite in the thin sections is easiest found in the 
places where a little cavity is cut through. In these many of 
the little hexagonal tablets or fragments of them appear. In 
the paste it is not so easy to find it, because its absolutely 
colourless behaviour makes it disappear between the other con- 
stituent minerals. ‘There it forms the characteristic tile-like 
agoregations, first of all described by Zirkel, and then found in 
many rocks. To see it, it seems useful to change the position of 
the tube, screwing it higher and deeper, adapting a high magni- 
fying power. Examining some of the largest hexagonal tablets of 
tridymite in the cavities, under crossed Nicols, I made the obser- 
vation that they do not remain, when turning them in the plane 
of the object, dark, as should be the case, if they were hexagonal 
and uniaxial, but that they show a maximum of light and extine- 
tion four times. In the very thin tablets the difference between 
lizht and dark is not very great, but always very observable. 
Also the tablets show that they have not equal optical 
orientation in every part of a tablet; I did not find one of 
them of asimple behaviour in that point. Some of them are com- 
posed of light and dark places in a somewhat chess-board-like 
* Comptes rendus 1875, Ixxx. 364-371 and 1876, Ixxxii. 1017-1022. 
