370 Proceedings of the Royal Society. 
he had discovered in Topaz, the author observed certain optical phe- 
nomena, depending on a peculiarity of structure. This peculiarity is 
manifestedeither in the depolarisation of light, when it gives rise to four 
quadrants of light, separated by the radiiof ablack rectangular cross si- 
milar to the central portion, or the tints of the first order in the uni- 
axal system of polarised rings, or in the unequal refraction of com- 
mon light, which gives rise to the mirage of a luminous point, in the 
form of concentric circles surrounding the centre of force. In every 
case there was found a quadrangular cavity in the centre of the in- 
tersection of the cross, generally dark and opaque, but in one case 
having a luminous spot in the centre. These cavities are from the 
sooo to the 7,55 of an inch in diameter. 
These cavities are quite distinct from all those formerly described 
by the author ; and from the phenomena above described, he con- 
cludes that the contents of each cavity have exerted an elastic force 
on the surrounding mineral while in a plastic state. In some cases 
fissures are seen proceeding from the central cavities, but these are 
supposed to have been produced after the mineral had become indu- 
rated, and had already been subjected, in the plastic state, to the 
pressure or force above indicated. 
These cavities never accompany the cavities with two fluids, but 
occur in specimens containing numerous embedded erystals, differ- 
ing little from Topaz in refracting power. 
Since the mineral must have been plastic when it yielded to the 
pressure here noticed, it cannot have been formed by the aggrega~- 
tion of molecules having the primary form of the crystal. 
These considerations, along with others connected with the crys- 
tals, which oceur in the cavities of Topaz, have led the author to 
adopt the idea of a new and peculiar kind of crystallization, to which 
he will soon direct attention. 
2. Extracts from Letters to the General Secretary, on the 
Analogy of the Structure of some Voleanic Rocks with 
that of Glaciers. By C. Darwin, Esq., F.R.S. Specimens 
were exhibited. With Observations on the same sub- 
ject, made by Professor Forbes. 
“I take the liberty of addressing you, knowing how much you 
are interested on the subject of your discovery of the veined struc- 
ture of glacierice. I have a specimen (from Mr Stokes’s collection) 
of Mexican obsidian, which, judging from your description, must 
resemble, to a considerable degree, the zoned ice. It is zoned with 
quite straight parallel lines, like an agate; and these zones, as far 
as I can see under the microscope, appear entirely due to the greater 
or lesser number of excessively minute, flattened air cavities. I can- 
not avoid suspecting that in this case, and in many others, in which 
