
ParriIl.§ii] | ROCK-FORMING MINERALS. 67 
_ - substance of fossil wood (wood-opal). Several forms of opal are deposited 
_ by geysers, and are known under the general appellation of sinters, 
Hydrated silica appears likewise as the result of plant and animal 
erowth in tripoli powder, randanite, and other earths which are 
composed mainly or wholly of the remains of diatoms, &e. 
Corundum occurs in clear rhombohedral forms (sapphire and 
tuby); also in dull, coarse, feebly translucent crystals (corundum), 
and in an amorphous granular form mixed with iron oxide (emery). 
HH. 9. Gr. 3°9—4. Alumina or aluminic oxide, Al, O, = Al 53:2 
0468. Found in crystalline rocks, particularly in certain serpentines 
and schists, gneiss, granite, dolomite, and rocks of the metamorphic 
series. The largest deposits of corundum yet known occur in the 
eastern states of America, from Massachusetts to Alabama. One of 
these runs for four miles, witha thickness of four feet, in a talcose 
_ slate and serpentine between gneiss and mica-slate in the centre of 
the Green Mountains. The occurrence of such enormous masses of 
alumina has been pointedly dwelt upon by Dr. F. A. Genth, who 
has brought to light a remarkable series of transformations of 
corundum into other minerals, among which are spinel, zoisite, 
felspars, tourmaline, fibrolite, cyanite, chlorite, lazulite, and the micas 
known as damourite and margarite. He affirms that large beds of 
corundum associated with the deposition of chromiferous chrysolite 
beds (since altered into serpentine) have been subsequently acted upon 
in such a way as to be converted into the minerals just mentioned, 
and that a portion of the altered products remains as large beds of 
mica- and chlorite-slates or schists. The difficulty of explaining how 
such alterations could take place in a substance which in our 
laboratories so resists solution, he has not yet been able to solve.? 
Corundum (sapphire and ruby) has been formed artificially. 
Tron Oxipes.—F our minerals, composed mainly of iron oxides, 
occur abundantly as essential and accessory ingredients of rocks. 
Hematite, Limonite, Magnetite, and Titanic iron. 
Hematite (ler oligiste, Rotheisen, Hisenglanz) occurs crystallized. 
in rhombohedral forms with splendent metallic lustre (specular iron) 
but most commonly in compact or crypto-crystalline, usually fibrous, 
sometimes amorphous aggregations (red iron), with cherry-red streak. 
H. 55—6°5. Gr. 519—5-28. Ferric oxide, sesquioxide or peroxide 
of iron, Fe, O,—Fe 70, O 80. In the crystallized form the mineral 
occurs In veins as well as lining cavities and fissures of rocks. The 
fibrous and more common form (which often has portions of its 
mass passing into the crystallized condition) lies likewise in strings 
or veins; also in cavities, which, when of large size, have given 
opportunity for the deposit of great masses of hematite, as in 
1 American Phil. Soc, 1873. 
? But of the reality of some of the remarkable metamorphisms he describes, the 
present writer can speak with the confidence arising from a personal inspection of the 
proofs with which Dr. Genth favoured him at the Laboratory of the University of 
Pennsylvania in, October, 1879. ; 
F 6 
