“ParrIl.§ii] ROCK-FORMING MINERALS. ——69 
able size; in other schists and in crystalline massive rocks like 
- granite, in diffused grains or minute crystals; also found in massive 
beds among schists and gneisses, as in Norway and in the eastern 
_ states of North America. One of the essential ingredients of basalt 
\ 
and other volcanic rocks, being there present in minute octohedral 
erystals, or in granules or crystallites. Likewise found as a pseudo- 
morphous secondary product resulting from the alteration of some 
previous mineral, as hematite, pyrite, quartz, hornblende, augite, 
garnet andsphene. This mineral may thus result from either aqueous 
or igneous action. It has likewise been observed with hematite, &c., 
as a product of sublimation at volcanic foci where chlorides of the 
metals in presence of steam are resolved into hydrochloric acid and 
anhydrous oxides. 
Magnetite is liable to weather by the reducing effects of decom- 
posing organic matter, whereby it becomes a carbonate and then 
by exposure passes into the hydrous or anhydrous peroxide. The 
magnetite grains of basalt rocks are very generally oxidised at the 
surface, and sometimes even for some depth inward. Michel-Lévy 
has observed them to be enveloped in biotite.’ 
Titanie Iron (Titaniferous Iron, Menaccanite, Ilmenite, Fer 
titané, Titaneisen), distinguished from magnetite by its rhombohedral 
erystallization, occurs frequently in thin plates or tables, as well as in 
diffused grains. H. 5—6. Gr. 4:5—52. <A mixture of oxides of 
iron and titanium in considerably variable proportions, being some- 
times an isomorphous mixture of titanic acid and ferrous oxide; 
sometimes with the addition of ferric oxide, or with that of mag- 
nesium titanate. Scarcely to be distinguished from magnetite when 
seen in small particles under the microscope, but possessing a 
brown semi-metallic lustre with reflected hght; resists corrosion by 
acids when the powder of a rock containing it is exposed to their 
action, while magnetite is attacked and dissolved. Occurs in 
scattered grains, plates, and crystals as an abundant constituent 
of many crystalline rocks (basalt rocks, diabase, gabbro, and other 
igneous masses); also in veins or beds in syenite, serpentine, and 
metamorphic rocks. Some of the Canadian masses of this mineral 
are 90 feet thick and many yards in length. 
Titanic iron frequently resists weathering, so that its black glossy 
eranules long project from a weathered surface of rock. In other 
cases it is decomposed either by oxidation of its protoxide, when the 
usual brown or yellowish colour of the hydrous ferric oxide appears, 
or by removal of the iron. The latter is believed to be the origin of 
a peculiar milky white opaque substance, frequently to be observed 
under the microscope, surrounding and even replacing crystals of 
titanic iron, and named Leucoxene by Giimbel.? 
MANGANESE OXIDES are frequently associated with those of iron 
1 Bull. Soc. Geol. France, 3rd ser. vi. p. 164. 
2 Die paldéolitische Eruptivgesteine des Fichtelgebirges, 1874, p. 29. See Rosenbusch, 
Mik. Physiog. ii. p. 336. De la Vallée Poussin and Renard, Mem. Couronnées Acad. Roy. 
de Belgique, 1876, xl. Planche, vi. pp. 34 and 35. Fouqué et Michel-Lévy, op. cit. p. £26. 
