>. tyr tees 
= ae 
~ er \ : 
_ a - 
. ee has 
ParrIl.§ii] | ROCK-FORMING MINERALS. 65 
basalts of Bohemia and Greenland. Nordenskidld observed that 
at the same locality in Disco Island, where he found the large 
blocks of native iron, the underlying basalt contained lenticular and 
_ dise-shaped blocks of precisely similar iron. He infers that the 
whole of the blocks may belong to a meteoric shower which fell 
during the time (Tertiary) when the basalt was poured out at the 
surface. He dismisses the suggestion that the iron could possibly 
be of telluric origin. But the microscope reveals in this basalt the 
presence of minute particles of native iron which, associated with 
_ viridite, are moulded round the crystals of labradorite and augite.? 
Daubrée appears therefore to be justified in regarding this iron as de- 
rived from the inner metallic portions of the globe which lie at depths 
inaccessible to our observations, but from which, on his view, the vast 
_ Greenland basalt-eruptions have brought up traces to the surface.® 
Tn the great majority of cases the Oxides occur combined with 
some acid. A few uncombined take a prominent place as essential 
constituents or frequent ingredients of rocks. 
Siica is found in three forms, Quartz, Tridymite, and Opal. 
Quartz occurs either (1) crystallized in clear hexagonal prisms 
(rock-crystal, amethyst, cairngorm), also opaque or translucent, 
granular, crystalline (common quartz, vein quartz), or (2) non- 
erystalline, crypto-crystalline, or compact (calcedony, hornstone, 
jasper), often coloured with iron or otherimpurity. Si O,=Si 46°67, 
O 53°33. H. = 7. Gr. 2°5—2°8. Calcedony includes translucent, 
compact, non-crystalline minerals occurring in stalactitic or en- 
crusting forms, and in nodules and layers: regarded as intimate 
mixtures of amorphous (soluble in caustic potass) and crystalline 
silica. 
Quartz is abundant as (1) an essential constituent of rocks, as in 
granite, (p. 131), gneiss, mica-schist, quartz-trachyte, quartz-porphyry, 
sandstone; (2) an accessory ingredient filling wholly or partially 
veins, joints, cracks and cavities. It has been produced from (a) 
igneous action, as in volcanic rocks; (b) aquo-igneous or plutonic 
action, as in granites, gneisses, &c.; (¢) solution in water, as where 
it lines cavities or replaces other minerals. The last mode of forma- 
tion is that. of the crystalline and non-crystalline quartz and 
calcedony found as secondary ingredients in rocks. 
The study of the endomorphs and pseudomorphs of quartz is of 
great importance in the investigation of the history of rocks. No 
mineral is so conspicuous for the variety of other minerals enclosed 
within it. In some secondary quartz crystals each prism forms 
a small mineralogical cabinet enclosing a dozen or more distinct 
minerals, as rutile, hematite, limonite, pyrites, chlorite, and many 
others. Quartz may be observed replacing calcite, aragonite, 
1 Geol. Mag. ix. ? Fouqué et Michel-Lévy, op. cit. p. 443. 
5 Daubrée, Discours, Acad. Sciences. 1 March, 1880, p.17. See also W. Flight in 
Geol. Mag. ii. (2nd ser.) p. 152. 
* See Sullivan, in Jukes’ Manual, p. 61. 
F 
