ON THE EFFECTS OF LONG-CONTINUED HEAT. 183 



same time silicates were formed, hydrated or anhydrous, according to the 

 degree of heat ; when fragments of obsidian were inserted, crystals of 

 Rhyacolite appeared ; and the silicated water of Plombiere being substituted 

 for plain water, and kaolin for obsidian, crystals of diopside insinuated 

 themselves into the silicated substance of the tube, and the kaolin was 

 changed into a substance possessing felspathic characters. 



All these experiments are adverse to the idea that the primary rocks have 

 undergone fusion. The best natural criterion, perhaps, of the temperature 

 at which they Avere formed, was afforded by the discovery, in 1828, of a 

 method of manufacturing ultramarine, based on Vauquelin's identification 

 of a furnace-product with the Lapis lazuli found in granite and in primitive 

 limestone. In some specimens which I possess of the latter rock, this 

 beautiful mineral may be seen enamelling with minute specks, and with 

 perfect distinctness, within and without, all the plates of the calcareous 

 crystals, which are here and there interspersed with small crystals of sulphate 

 of lime. The heat at which the artificial ultramarine is made is that of red- 

 ness. A lower temperature will not suffice to produce the colour, and a 

 higher destroys it. 



We can now better understand how Hunterite, a white felspathic mineral 

 containing 1 1*6 per cent, of water, can have been formed where it is found ; 

 a hydrated silicate of alumina in the bosom of molten granite is an anomaly 

 for which high pressure would scarcely account; but if the rock was at the 

 temperature only of a low red heat, the formation of this mineral, and of 

 the hydrated micas, will no longer appear a marvel. 



Other notices of ancient degrees of heat have been observed in the 

 strata. In a cavity within a quartz crystal from Dauphine, Davy found a 

 viscous inflammable fluid in small quantity, in a perfect vacuum*. In the 

 cavities of other quartz crystals he found water and rarefied air. Sorby, 

 having determined the amount of rarefaction in one such from a bed of 

 mica-slate, in which he detected many others, calculated the temperature of 

 the crystal at the time of its formation to have been 320° Fahr. In one case 

 Davy found evidence of pressure which had condensed the elastic fluid in a 

 crystal of quartz, and Brewster observed the like in crystals of topaz. 



From a general review of the researches now detailed, the following infer- 

 ences may be drawn : — 



1. That all the consolidated strata, viewed chemically, bear marks of sub- 

 jection to an action of heat agreeable to the theory of the earth's refrigera- 

 tion, in direct proportion to the age of their deposit; and that they show that 

 action most explicitly in the presence, throughout, but more abundantly as 

 the series descends, of that peculiar form of silica which is chemically repro- 

 duced by the action of heated volatile matter. 



2. That the igneous minerals were formed by molecular aggregation, at a 

 heat not exceeding, perhaps, that of an ordinary fire, either as a residuum 

 from the expiration of fusible and volatile materials, or more generally as a 

 deposit from volatile forms of matter. 



As there are two classes of eruptive rocks, the quartzose and unguartzose, 

 so there are two classes of emanation which accompany them, and deposit 

 earthy minerals, differing for each class, in the neighbouring strata. They 

 generally mantle round the rock, and but seldom penetrate it ; as if it had 

 rather made room for them to rise, than as if they made part of its substance. 

 Yet they bear a resemblance to the character of the rock which they follow. 

 Thus the crystallized oxide of silicon is the characteristic ingredient of granite 



* Rose quartz from granite, and cornelian from trap, are coloured by a carburet of hydro- 

 gen ; crystals of graphite also have been found in quartz ; but as carbonic acid must have 

 existed before plants could grow, these facts are no proofs of antecedent organic structure. 



