16 B 
Proceedings of Poijal Society of Pdinhurgh. [fee. 20, 
Finally, I have to call attention to the quartz. This mineral 
exhibits most peculiar properties. In some specimens of the diabases 
(viz., Hound Point, 8 feet above the contact with a fragment of 
sandstone, which is enclosed about 4 feet above the base of the bed, 
and 15 inches from the junction with an included fragment of sand- 
stone) there appear (most clearly seen during the preparation of the 
slices) quartz grains, surrounded by sharply contoured hexagons. 
An examination with the naked eye, however, suffices to show that 
these hexagons are sometimes filled up witli calcite substance, and 
that in most cases their central portion only represents a round grain 
of quartz, the hexagonal outline being given by calcite which 
surrounds the quartz grain. I have only once been able to find a 
hexagon that was filled throughout with quartz substance represent- 
ing a true quartz crystal. It seems possible that all these hexagonal 
outlines were in former times due to automorphic quartz crystals, 
which no longer occupy all the space they may have filled before. 
Being unable to offer a sufficiently clear explanation of these, I have 
contented myself with describing them. In the sliced specimen the 
sharp outlines of the hexagons have disappeared. With regard to the 
quartz substance itself, we find that between crossed nicols it exhibits a 
peculiar structure. It might fairly be supposed that, between crossed 
nicols, a section of a simple crystal should exhibit the same colour 
in all parts. We are therefore astonished to see the rounded quartz 
grains which I have described, variously coloured along their radii, 
or even consisting of several grains, each of which exhibits a homo- 
geneous optical orientation. The former phenomenon might be taken 
for the spheroidal structure of chalcedony, unless the quartz sub- 
stance enclosed Sorby’s “stone-cavities,” and gave no interference 
figure between crossed nicols. The latter phenomenon, however, 
might be interpreted as the combination of a right-handed quartz 
with a left-handed one, unless the single grains within one hexagon 
exhibited, between crossed nicols, more than two different colours. 
It remains to answer the possible objection that several quartz 
crystals could have grown together with parallel crystallographic axes. 
This interpretation is refuted by tlie fact that all the hexagons are 
always sharply contoured and regularly formed. Therefore we 
cannot but assume that the quartz exhibits an anomalous pheno- 
menon of polarisation, hitherto unnoticed, at least as far as my 
