PROCEEDINGS OF GEOLOGICAL SOCIETIES. 
71 
These colours are due to a peculiar structural arrangement, and may be 
explained by the laws of optics. And if any one will examine the opal 
from Mexico and the substance prepared by evaporation in vacuo by 
Professor Graham's experiment, he cannot fail to be struck with the 
resemblance between the two. The mineral termed " h}- alite " is also a 
kind of opal met with in basaltic rocks. It is another form of amorphous 
silica. This hyalite contains an amount of water, the extremes of which 
are 3 and 6 per cent. 
The processes carried on in our blast furnaces or iron-smelting furnaces 
on so large a scale in various parts of this country, may really furnish indi- 
cations of great importance to the geologist. In the hearths of blast 
furnaces is occasionally found a white, delicate, fibrous substance, to which 
the name of "fibrous silica" has been given. It has been carefully ex- 
amined, especially by Uose, who finds it to consist essentially of silica. 
It is silica in the amorphous state, produced at a high temperature, and, 
therefore, having a specific gravity not exceeding 2*2 or 2"3. We are not 
perfectly certain yet as to the precise conditions under which it has been 
generated, but most likely it may have resulted from the oxidation of 
sihcon. Sorby informs us that he obtained fibrous silica, exactly similar 
to that occurring in the hearts of blast furnaces, by passing fluoride 
of silicon, together with the vapour of water, through a porcelain tube 
heated to red-whiteness. Ey introducing the fluoride of silicon at one end 
of the tube, and the steam at the other, he obtained silica only in small 
vitreous grains. That is the amorphous silica. 
In concluding this lecture, the lecturer said he could not help forestalling 
what he should say hereafter as to the formation of certain igneous rocks, 
especially of granite. For along time it has been the received notion, that 
all granite, which occurs so abundantly in the crust of the earth, has been 
the result of igneous fusion at a very high temperature ; but there are cer- 
tain difficulties which have always been in the way of accepting this view 
of the subject, — difficulties known, at all events, to those who have been 
accustomed to make experiments on the fusion of mineral substances at 
high temperatures. 2s'ow, let us look at the fact of quartz occurring 
in this granite. Granite consists, as most of us know, of three minerals — • 
quartz, mica, and felspar; and the quartz in it is crystallized, and always 
has the specific gravity 2'6. There is not a single instance known to the 
contrary. There is therefore reason to believe that that quartz never 
could have been fused ; for we have seen that the moment we fuse silica, 
no matter in what state it was previously, we obtain a glass-like colloidal 
or non-crystalline mass, having a specific gravity never exceeding 2 3. 
In this fact he thought it would be agreed that there is something like a 
foundation for the inference (even from this single fact) that .such granite 
could never have been produced under the condition of a hif/h tempera- 
ture. What those conditions were under which it was produced would be 
hereafter considered. 
Manchester Geological Society. — 2\ih November, 1863. — A new 
safety-hook was described by Mr. George Wild, and the results of an at- 
tempt to get information from coal-proprietors twenty-four years ago, was 
brought before the meeting by Mr. Binney. It appears that to 2o() circu- 
lars by the Society, only one reply was received, — from Mr. Ilalph Thick- 
nesse, of Wigan. 
