180 
THE GEOLOGIST. 
— a silicate of zirconia. It occurs, Deville saya, in octaliedral crystals, and 
these crystals exactly resemble those of Monte Somma, in Vesuvius, He 
assures us that they have the same faces, the same angles, aud 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 ^re. Every mineralogist kno"v\"s tliat certain specific minerals have pecu- 
liarities according to the locality, and these must depend upon certain con- 
ditions attending their formation. Kow, 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 tliat 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 fiuor, 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. 
AYlien 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 SNaFl -f AlgFlg. 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 la3^er of gneiss, and in the vicinity of mica. According to 
Bischoff", 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 as- 
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 fluor spar 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. Eecently, some important experiments have been made 
by "VVohler 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. 
