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 " hyalite " 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 Hose, 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 

 silicon. 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. By 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. JNTow, 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 high tempera- 

 ture. What those conditions were under which it w r as produced would be 

 hereafter considered. 



Manchester Geological Society. — 24th 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 250 circu- 

 lars by the Society, only one reply was received, — from Mr. Balph Thick- 

 nesse, of Wigan. 



