DECOMPOSITION OF ROCKS YEINS. 
o 
DECOMPOSITION OF ROCKS- 
VEINS. — According to Becqueiel, veins are 
not to be considered as products of one general 
cause, but of a concurrence of several causes. 
The viens in the most ancient rocks are smaller 
than in the newer rocks, the largest existing 
in the schists and transition limestones. Wer- 
ner considered that rocks were decomposed 
by two acids : I. By carbonic acid as 
when granite and gneiss or felspar alone are 
decomposed and form kaolin, 2. Sulphuric 
acid derived fiom pyrites, as in veins of fels- 
p r, mica, and amphibole. Arsenic acid 
he considered produced a similar effect. 
M. Fournet, who has paid much atten- 
tion to veins, distinguishes two kinds: those 
of igneous origin, such as porphyries, tra- 
chites, &c. in which the silica has formed 
combinations by means of heat ; and those 
of aqueous origin, as we see illustrated in 
mineral waters. To exemplify the former 
he cites those instances where sulphuret of 
iron, silica, and iron pyrites have been depo- 
sited upon the fragments of primitive rocks, 
and with regard to the latter, he mentions 
cases where talc and mica are changed into 
a grey substance, and granites where felspar 
is altered into kaolin, likewise talcose schists 
where steatite is isolated in veins. In the 
veins of Pont Gibaud, he observe.d four other 
epochs. At the second period new branches 
were formed, which were filled with secondary 
and tertiary products, especially quartz, 
but likewise sulphurates, which have formed 
alternating zones of pyrites, galena, and 
hyalines quartz in small crystals. A third 
period distinguishes a dilatation which disturb- 
ed the sources of the galena and introduced 
solutions of sul pirates of barytes- At the 
fourth epoch, the inerusting power of these 
sources appears to have been enfeebled, when 
parites and minute veins of carbonates were 
deposited. The fifth epoch was contempo- 
rajaeous with the basaltic eruptions. It is 
obvious, that for an explanation of the mode 
in which these veins are filled, we must have 
recourse to chemistry. Thus, hydrate of iron 
proceeds from the decomposition of pyrites; 
the powder of hydrous oxide is derived from 
the decomposition of the carbonate, ga ena 
is gradually converted into a black pulveru- 
lent substance, which gives birth to black 
and white carbonate. With regard to the 
formation of rock-salt, Dumas has observed 
that in one variety of it which decrepitated 
when placed in water, the cause was attri- 
butable to hydrogen which condensed in its 
cavaties, 
GRANITE, — Saussure attributed the de- 
composition of this rock to a corrosive juice 
which dissolved the gluten uniting all its 
parts. Vanquelin and Alluan traced the 
cause to disintegration of the rock, and the 
removal of the alkali in the felspar by water. 
But Berthier has shewn that silica as well as 
potash is removed, a silicate of potash disap- 
pearing and silicate of alumina remaining. 
Felspar is probably one of those bodies whose 
particles are placed in such intimate union 
that acids have no effect upon it until it be 
exposed to electro-chemical agency. Four- 
net has observed three preliminary stages in 
the decomposition of granite, 1. A superior 
zone of a red or yellow colour, indicating the 
peroxidation of iron, 2. A middle zone of a 
deep green colour. 3. An inferior zone, pre- 
senting all the characters of a perfect granite, 
but falling to pieces when touched. Me ac- 
counts for the successive decomposition from 
the surface, internally to dimorphism, which 
has changed their crystalline texture like ar- 
ragonites and laumonites, Gustav. Rose 
has produced pyroxene and amphibole as in- 
stances of this dimorphism, of which some 
result from rapid, others from slow cooling. 
The theory of the felspar decomposition Four- 
net sums up shortly, 'i’he iron is peroxidized, 
carbonic acid is absorbed and takes the 
place of the silica, which, being set at liberty 
in a gelatinous state, dissolves in water, or 
alkaline carbonates, and gives origin to cry- 
stals of hyaline quartz, iorites, agates, opal, 
calcedony, and silicates, as chabasite, me- 
sotype. 
I'his theory, however, rests upon two sup- 
positions which have not yet been demonstrat- 
ed. 1. That igneous rocks do not acquire a 
state of permanent equilibrium, and that they 
exhibit in the course of time an effect of di- 
morphism, and 2. 'fhat carbonic acid is ab- 
sorbed by tliese rocks. The latter appears to 
be strongly exhibited in Auvergne, where 
numerous mineral springs, which escape from 
granite fissures, act upon the rocks, and form 
small irregular basons which they fill with 
hydrous peroxide of iron. 
SPARRY IRON ORE.-Granite before 
it decomposes disintegrates, but the iron ore 
retains its form, and yet changes its chemical 
nature. Becquer’el has examined the process 
of the decomposition of this mineral in Isere, 
and he has found it entii'e when preserved 
from the contact of air and water. In Dau- 
phine it is decomposed in such a manner as 
to give out heat and light, which burst into 
flame and continue to burn. The inhabitants 
regard the presence of these flames as a de- 
cided proof of the existence of rich mines of 
this mmeral. The mineral contains carbonate 
of manganese and magnesia. The iron and 
manganese change into hydrates, lose their 
carbonic acid which combines with the mag- 
nesia, and renders it soluble in water. Water 
is decomposed to afford oxygen to the hydrate, 
and the hydrogen inflames after overcoming 
an immense pressure. 
According to Chapert, when some of the 
minerals accompanying this iron ore are 
roasted, and left to spontaneous action, after 
some days, sulphate of magnesia and iron, 
and carbonate of copper appear, facts of great 
importance in electro-chemistry, Four kinds 
of pyrites accompany this ore, which give 
origin, to 1. Neutral sulphate of iron. 2. 
Earthy sulphate, a yellow substance, resinous 
or earthy, 3. Ochre proceeding from the ac- 
tion of air upon the neutral sulphate ; besides, 
sulphate of iron and alumina, manganese, 
line, zinc, &c. 
, LAVAS, — Granite decomposes readily in 
contact witli bay-salt, as is evinced in Scot- 
land and Clermont. The facility of the de- 
composition of lavas varies with their com- 
position ; thus the pyroxenic rocks of Au- 
vergne decay more rapidly than the Labra- 
