180 



THE GEOLOGIST. 



— a silicate of zirconia. It occurs, Deville says, in octahedral crystals, and 

 these crystals exactly resemble those of Monte Somma, in Vesuvius. He 

 assures us that they have the same faces, the same angles, and the same ex- 

 ternal characteristics ; so that he concludes that it is almost certain that 

 they must have resulted, from the same process — from the same operation 

 of fire. Every mineralogist knows that certain specific minerals have pecu- 

 liarities according to the locality, and these must depend upon certain con- 

 ditions attending their formation. JSow, as these crystals of zircon possess 

 such an assemblage of characteristics, we may reasonably admit that 

 Deville's conclusion has something like a good foundation to rest upon. 

 Then he further remarks, it may be demonstrated that the small quantities 

 of fluor existing in the metamorphic rocks, or rather beds of this kind, have 

 sufficed to form indefinite quantities of zircon. We shall, by-and-by, 

 direct attention to the subject of fluor, which may have played a very im- 

 portant part in the economy of nature, — a much more important part than 

 many persons are yet disposed to admit. It is, as we shall see, a very 

 .widely-diffused element, though occurring only in small quantity. 



When the same experiment was made entirely with zirconia instead of 

 alumina, the whole contents of the tube were completely transformed into 

 zircon. Zircon occurs in rocks which, there is reason to believe, have been 

 exposed to a tolerably high temperature. It occurs in the syenite of Nor- 

 way, for example, replacing felspar ; so that the rock consists only of 

 hornblende, zircon, and a small quantity of quartz ; hence the rock is 

 designated " zircon-syenite." 



The next mineral which it may be interesting to examine is the mineral 

 termed " cryolite," which, in fact, forms a geological bed. It is an ex- 

 ceedingly remarkable mineral, fusible at a comparatively low temperature. 

 It is a compound of fluoride of sodium and fluoride of aluminium. Its 

 formula is 3!N"aFl -f A1 2 F1 3 . It is an anhydrous mineral — that is, free 

 from water. It contains about 13 per cent, of aluminium. It is found in 

 Greenland in a layer of gneiss, and in the vicinity of mica. According to 

 Eischoff, there is a quantity of mica about it, which he supposes has played 

 a very important part. The bed of cryolite is eighteen feet thick. Cryolite 

 is associated with various minerals which undoubtedly are of aqueous 

 origin. For example, iron pyrites — though certainly not prepared at a 

 high temperature — copper pyrites, galena, and sparry iron ore, which cer- 

 tainly never could have occurred at a high temperature, — these are the asr 

 sociates of cryolite, and they tell us the story of its formation. It is clearly 

 produced by the agency of water as a solvent. It may be produced by 

 melting together directly fluoride of aluminium and fluoride of sodium. It 

 may also be formed in the wet way by digesting fluoride of sodium with 

 excess of hydrochloric acid and common gelatinous alumina. 



Bischoff supposes mica to have played a very important part as a source 

 of fluorine, — indeed, as a source of fluorine in common fluorspar which we 

 meet with in so many localities. 



The next subject for our consideration is one of considerable importance 

 — of the highest importance ; it is that of calcium and lime. 



All lime, like alumina, contains a metallic base. Calcium, the base of 

 lime, is exceedingly light, has a yellowish colour, is readily fusible, and 

 ..exceedingly oxidizable, so that it is impossible to expose it to the air with- 

 out its undergoing oxidation — contrary to what we have seen is the case 

 with aluminium. Recently, some important experiments have been made 

 by Wohler on the subject. He has discovered certain combinations of car- 

 bon and calcium equivalent to those known in iron — in the form of pig iron. 



