
Parr IL§ii]  ROCK-FORMING MINERALS 63 
chemical changes which rocks undergo from the action of pereo- 
lating water (Book III. Part II. Section it, § 1 and 2); and lastly, 
characteristics or peculiarities of weathering, where any such exist 
that deserve special mention. 
The native elements are comparatively of rare occurrence, and 
only two of them, carbon and sulphur, occasionally play the part of 
noteworthy essential and accessory constitutents of rocks. A few of 
the native metals, more specially copper and gold, now and then 
appear in sufficient quantity to constitute commercially important 
ingredients of veins and rock-masses. : 
| Graphite.—Rarely crystallized in hexagonal forms, usually 
granular, scaly, or compact. H. 0-5—1:0. Gyr. 1:9—2:3. Nearly 
pure carbon, but generally with at least 1 or 2 per cent. of silica, 
lime, iron, or other impurity. Under the microscope, opaque; ap- 
pearing velvet-black with reflected light. Found chiefly in ancient 
erystalline rocks, as gneiss, mica-schist, granite, &c.; some of the 
Laurentian limestones of Canada being so full of the diffused 
mineral as to be profitably worked for it; in rare instances coal 
has been observed changed into it by intrusive basalt (Ayrshire). 
Probably in most cases the result of the alteration of imbedded 
organic matter, especially remains of plants; occasionally observed 
as a pseudomorph after calcite and pyrites, and sometimes en- 
closing sphene and other minerals.* es | 
Graphite is little affected by percolating water, hence it is not a 
replacement mineral. But Vom Rath has described an example 
from Westphalia where calcite has been partially replaced externally 
by an encrusting pseudomorph of graphite.” 
_ Sulphur.—Crystallized in rhombic pyramids ; but more commonly 
compact, granular, powdery, stalactitic, or incrusting. H. 15—2°5. 
Gr. 1:9—2:1. Normally pure sulphur, but often much mixed with 
earthy, calcareous, or bituminous impurities. Occurs under two 
conditions. lst, as a product of volcanic action in the vents and 
fissures of active and dormant cones. Volcanic sulphur is formed 
from the oxidation of the sulphuretted hydrogen, so copiously 
emitted with the steam that issues from volcanic vents, as at the 
Solfatara, near Naples. It may also be produced by the mutual 
decomposition of the same gas and anhydrous sulphuric acid. - 2nd, 
in beds and layers or diffused particles resulting from the alteration 
of previous minerals, particularly sulphates, or from deposit in water 
through decomposition of sulphuretted hydrogen. The frequent 
erystallization of sulphur shows that the mineral must have been 
formed at ordinary temperatures, for its natural crystals melt at 
238:1° Fahr. Its formation may be observed in progress at many 
sulphureous springs, where it falls to the bottom as a pale mud 
through the oxidation of the sulphuretted hydrogen in the water. 
It occurs in Sicily, Spain and elsewhere, in beds of bituminous 
1 Vom Rath. Sitzwngsber. Wien. Akad. x. p. 67; Sullivan in Jukes’ Manual of Geology, 
8rd edit. p. 56. 
2 Neues Jahrb. M'n. 1874, p. 522. 
