68 - GEOGNOSY. — [Boo Il. 
cavernous limestones (Westmoreland). It occurs with other ores — 
and minerals as an abundant component of mineral veins, like- 
wise in beds interstratified with sedimentary or schistose rocks. — 
Scales and specks of opaque or clear bright red hematite, of 
frequent occurrence in the crystals of rocks, give them a reddish 
colour or peculiar lustre (perthite, stilbite). Under the microscope 
hematite is dull red or opaque, distinguishable from magnetite 
by crystallographic form and colour. It appears abundantly as 
a product of sublimation in the clefts of volcanic cones and lava 
streams. In veins and beds among rocks it is probably in most 
- cases a deposition from water, resulting from the alteration of some 
previous soluble combination of the metal, frequently the oxidation 
of the carbonate. It is found pseudomorphous after ferrous car- 
bonate, and this has probably been the origin of beds of red ochre 
occasionally intercalated among stratified rocks. It likewise re- 
places calcite, dolomite, quartz, barytes, pyrites, magnetite, rock-salt, 
fluor-spar, &e. 
Limonite (brown iron ore) occurs in no definite crystallized 
form, but in finely fibrous or indistinctly crystalline, mammillated, © 
encrusting, or stalactitic aggregates, often earthy and amorphous; — 
blackish brown to ochre yellow, with yellowish brown streak. H. 5, — 
Gr. 3°4—3'95. Consists of hydrous ferric oxide, Fe, 0, +8 H, O= 
Fe, O, 85°56, H,O 1444. Occurs in beds among stratified forma- — 
tions, and may be seen in the course of deposit through the action 
of organic acids in marsh land (bog iron ore) and lake-bottoms. 
(Book IV. Part II. Section iii.) In the form of yellow ochre — 
it is precipitated from the waters of chalybeate springs containing — 
green vitriol derived from the oxidation of iron sulphides.t Limo- 
nite is a common decomposition product in rocks containing iron 
among their constituents. It is thus always a secondary or deriva- 
tive substance resulting from chemical alteration. . 
The pseudomorphous forms of limonite show to what a large 
extent iron oxides are carried in solution through rocks. The 
mineral has been found replacing calcite, siderite, dolomite, 
hematite, magnetite, pyrite, marcasite, galena, blende, gypsum, 
barytes, fluor-spar, pyroxene, quartz, garnet, beryl], &e. 
Magnetite (I'er oxydulé, Magneteisen), isometric, abundant in 
octohedral forms, in crystallites, and in minute irregular grains 5 
also massive. Strongly magnetic. Black, with a semi-metallic lustre, 
and subconchoidal fracture. H 55—6°5. Gr. 4:9—5:2. Ferroso- 
ferric oxide—a mixture of ferrous oxide (Ie O 81:08) and ferric 
oxide (I*e, O, 68°97) or Fe 72:41; O 27°59, but often containing 
titanic acid or magnesia. Soluble in hydrochloric acid. Under 
the microscope distinguishable by its intense opacity, and by its blue 
black colour with reflected light. ' 
Occurs abundantly in some schists, particularly in chlorite-slate 
and talc-slate in scattered octohedral crystals sometimes of consider- 
' Sullivan, op. cit. p. 63. 

