John Pinkerton.

Petralogy. A treatise on rocks (Volume 1) online

. (page 13 of 35)
Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 13 of 35)
Font size
QR-code for this ebook


spar in the north of Scotland.

* It is sometimes classed with the Argillaceous, because some
kinds decompose into clay ; yet this effect probably arises from the
potash. But it forms the chief part of granite, which has never
been classed among argillaceous substances.

According to Lametherie, v. Q, felspar requires the most water
to crystallise, so must be the most ancient ; and is followed by horn-
blende, quartz, mica, magnesia, and the metals successively. But
the grey petrosilex of Vosges is a felsite. Ib. 352.



1,58 DOMAIN II. SILICEOUS.



STRUCTURE I. COMMON.

Aspect 1. Common foliated. This rarely forms
entire mountains, but such have been discovered
in Siberia. When it forms mountains it is gene-
rally white.

Aspect 2. Granular. Rocks of a fine white
granular felspar, resembling statuary marble or
dolomite. Sauss. 2144.

Aspect 3. Unctuous. Saussure, 1304, de-
scribes what he c&lhfelthspath gras, or unctuous
felspar, as having a visage more oily and trans-
lucent than common felspar : the fracture rarely
laminar; and the plates, when perceivable, not
being level, but often convex, so that almost all
the fractures are generally conchoidal. It is harder,
and less fusible, than the common.

STRUCTURE II. MINGLED.

This division may justly comprise numerous
rocks and entire mountains, consisting of felspar
interspersed with a little mica or quartz, or a few
garnets ; the preponderance of the felspar being so
great that they cannot be considered as granitels.



MODE III. FELSPAR.



159



Such rocks may be said to be always white, like
those consisting of felspar alone.

Felspar rock, with a few garnets, from a moun-
tain in the west of Scotland.

The same, lightly sprinkled with mica, from the
Alps.

The same, with a few grains of quartz, Petuntzc Petuntze.
of the Chinese, from Limoges, in France. The
fine porcelain of Sevre is composed of this sub-
stance, the quartz being carefully separated. The
same is also found in Cornwall, Saxony, China,
and many other countries. When the felspar is
decomposed into a white clay, it is called kaolin,
and is also necessary in the fabrication of porcelain.

The beautiful opalised kind of felspar, called
Labrador stone, is also a component part of rocks
in Finland and Norway, where it only reflects the
blue lustre ; both kinds, when viewed alone, with
the beautiful green felspar from Siberia, falsely
called the Amazon's stone (which Avas found near
the river of Amazons, in South America, and is a
jad), rather belong to gemmology, as they do not
constitute entire rocks. But the Labrador and
Norwegian rocks, considered as a compound, are
here classed in the Anomalous Domain.



Kaolin.



Labrador.



Green of
Siberia.



160 DOMAIN II. SILICEOUS.



MODE IV. FELSITE, OR COMPACT
FELSPAR.

Characters. Texture, Compact.

Hardness, felsparic. Fracture, minutely fo-
liated. Fragments, rather sharp, amorphous.

Weight, granitose.

Internal lustre, glimmering, or glistening.
Translucent, sometimes only on the edges.

Colours, various, as of felspar, which may also
distinguish it from keralite. Melts under the
blow-pipe.

It is doubtful if it form entire mountains; but
is an important rock, and among the most pri-
mitive.

Paiaiopetre. It is the Palctiopetre of Saussure, who found it
near the summit of Mont Blanc ; and a speci-
men which he sent to Lametherie, and which I
have seen, is compact felspar, easily fusible by
the blow-pipe.

P WaU S erius f ^ * s ^ e P et rosilex of Wallerius, and in conse-
quence of the Swedes and French ; so that it
must be carefully distinguished from real petro-
silex, which literally implies the rock-flint of the
Germans, being a modification of quartz, and
not of felspar. The name felsite was very pro-



MODE IV. FELSITE.

perly introduced by Mr. Kirwan; as a distinc-
tion between it and keralite seems first to have
arisen from an accurate examination of the
beautiful blue granite, discovered near Krieglach
in Stiria, where it occupies the place of common
felspar. The distinction, which is thus recent,
forms an important step in the knowledge of
rocks.

Wallerius sent a specimen of his petrosilex to
France, which I saw in jthe possession of Haiiy,
being rose-coloured compact felspar, from Sal-
berg. Saussure mentions a grey felsite, explored
like slates near Martigny, in the valley of the
Rhone; and the celebrated cascade of Pisse
Vache falls from a rock of this kind*. His j ad
is also now called compact felspar. Patrinf saw
in Siberia, near the celebrated silver-mine of
Zmeof, a mountain with singular natural forti-
fications, composed of felsite. They rise about
200 feet above the body of the mountain ; being

* 1046. Between Martigny and St. Maurice there is also a sin-
gular variety of rocks. Among them is a kind of petrosilex, grey,
hard, and sonorous, with a little transparency, which rises in thin
plates, perfectly flat and regular : hence it is used as a slate-quarry.
It is probably of the same nature with patrinite, or laminar felsite.
The rock of the famous cascade called Pisse Vache, seems of the
same kind, but approaching nearer to a feljad, greenish, and semi-
transparent. It melts like a felsite, but with greater difficulty.

f i. 134.

VOL. I. M



162 DOMAIN II. SILICEOUS.

on all sides as steep as a wall, and only pierced
with a few difficult openings. The summit forms
a platform, about 500 paces by 200, covered
with blocks and fragments of various kinds of
felsite, some laminar, others veined in zigzag.
Some have the triangular form of half a cube,
cut by its diagonal ; and one large mass is com-
posed of angular fragments of felsite, in a paste
of the same substance, so as to constitute a bricia.
When Dolomieu wrote, the knowledge of
rocks was far from having attained even the pre-
sent degree of precision ; which is however so
far from being perfect, that perhaps another en-
lightened century may elapse, before all the
rocks shall have been discovered, analysed, and
examined, so as to be reduced to their proper
domains and modes. The following rock, with
a base of his petrosilex, which is felsite, probably
belongs to this division, though he mentions it
after the toad-stones and variolites*.
F Corsica f " ^ must here mention some glandular stones
which I found in Corsica, chiefly in the valley
of Nidof, and which have petrosilex for a base;
none have appeared to me more curious nor
more instructive. The very fine paste, which

forms the ground of the mass, is of different



* Jourh. de Phys. 1794, p. 260, note,
t Niolo ?



MODE IV. FELSITE.

colours, white, grey, red, or brown. , The glo-
bules, whose growth may be traced from the
instant when, having a diameter of half a line,
they begin to be apparent, till they have acquired
an inch in size, are generally black, with an
aspect of horn, sometimes brown or red, but
always of a different colour from the base : they
are striated from the centre to the circumference,
and have almost always a small crystal of fel-
spar, or a grain of quartz, for a central point.
Sometimes the gland is not completely formed ;
its place is only marked by the circular spot
round the central grain. The greater part of
these glands adhere strongly to their base, per-
fectly incorporating with it : some, but not those
which are striated, may be detached, and leave
their impressions. It seems to be a mixture of
steatite which favours their separation. These
globules, which are not always harder than the
paste which contains them, are affected by fire
like petrosilex; and only seem to differ from
their base by the kind of texture that a peculiar
aggregation has caused them to assume."

Dolomieu, following the observations of Saus-
sure, 1079, has demonstrated*, that the petro-
silex of the Swedish and French authors is a

* Journal de Physique, new series, vol. i. f>. 244.

\T ^



DOMAfN II. SILICEOUS.



compact felspar. It is sometimes of a greenish
hue, from a small quantity of steatite dissolved
in its paste ; and sometimes grey or black, from
a mixture of bitumen, as Dolomieu argues, from
its becoming white before it melts. Felspar often
passes into felsite, and the latter often contains
little scales or crystals of the former.

This ingenious, but prolix and discursive
Tw fepar! f author, distinguishes two kinds of felspar; that
which contains lime, and that which contains
magnesia. The latter is more hard and weighty,
and less fusible than the other; and as it ap-
proaches to the nature of jad, might, by a com-
plex term, alike useful for precision and the
Feijad. memory, be called felj ad. Some granites present
both kinds, but felsite generally contains lime;
it is also found with crystals of .schorl, plates of
siderite, or veins of quartz.

According to Dolomieu, felsite forms the base
of several porphyries found in the valley of
Niolo, in Corsica. One kind is green, and as
fine as chalcedony, sprinkled with an infinite
number of red dots, being little crystals of fel-
spar. A brown felsite porphyry, of a schistose
kind, is used for slates in the village of Pergine,
in the bishopric of Trent.

" The mountains of Tyrol, between Trent and
Bolsano, are almost entirely composed of por-



MODE IV. FELSITE.



165



phyries, \vith a base of petrosilex of different
tints; and the little valley of Fierrozo, in which
flows the river Fersina, and at the entrance of
which is the village of Pergine, might be called
the valley of porphyries, from the infinite num-
ber of varieties which that rock there presents,
always preserving the same base. There are
grey, green, red, brown, black. There are some
similar to bricias, of a green cement with pieces
of other colours. The petrosilex in it gradually
passes to the granitic texture, and to the state of
schistose rock, without the beds changing their
direction."*

STRUCTURE I. COMMON.

Rose-coloured felsite, from Salberg, in Sweden.
This is interesting, as being the petrosilex of Wal-
lerius, which led to many errors of the Swedish
and French writers. He describes the following
kinds f.

A scaly, grey petrosilex, from Salberg.

A scaly brown, from Garpenberg.

The red scaly petrosilex, from Dalecarlia.

The green scaly petrosilex, from the same.

The blackish scaly, from Dannemora.

He then describes several specimens of a waxy

* Ib. 247, note. f i. 280.



DOMAIN II. SILICEOUS.



fracture ; and observes, that his petrosilex some-
times runs in layers in calcareous mountains ; so
that he has confounded the secondary hornstein
with the primitive felsite. When a stone passes
into jasper, it must properly belong to keralite or
rock-flint, which, like jasper, is infusible.



STRUCTURE II. LAMINAR.



This kind is observed by Saussure as already^
mentioned. It has been confounded by some with
keralite or hornstein.

The clink-stone of Werner, sometimes ridicu-
lously called basalte en tables by the French mi-
neralogists, may properly be classed under this
division, as having no connexion with the basaltic
modes, which are characterised by the abundance
of iron, but most intimate relations with felspar,
as will appear from the following analyses by
Vauquelin and Klaproth :

FELSPAR. KLINGSTEIN.

Silex . . 62,83 ..... 57,25

Argil . . 17,02 ..... 23,50

Lime . . 3,00 ..... 2,75

Potash .. 13,00 . Soda . . 8,10

Iron . . . 1,00 (manganese 0,25) 3,25

Loss . . 3,15 (water3) . 4,9

100 100



MODE IV. FBLSITE.



167



The klingstein porphyry of Werner, which he
also absurdly calls porphyry slate, is a schistose
felsite, with crystals of felspar; as it happens in
many substances that an earthy and compact base
is spotted with crystals of the same substance, the
forms of some of the molecules having disposed
them to crystallise, while the others became sedi-
mental. The klingstein porphyry is generally black
or grey, but sometimes of a green, yellow, or
brown tinge, like the klingstein, which seems to
admit most of the colours of felsite ; but is of a
schistose texture, and contains little cavities, often
lined with quartz crystals, much resembling lava
with a basis of felsite.

As the French seem first to have observed this
rock, it may be called patrinite, in honour of Pa- Patrinite.
trin, an eminent French Geologist and Volcanist.
It sometimes contains zeolite, calcareous spar, and
crystals of siderite. Mr. Jameson has observed
that it is sometimes vesicular*, or full of little
cavities, " but not so mueh so as basalt." Part
of his description of patrinite may be transcribed.

" 2. It occurs sometimes in tables and columns:
also in veins that traverse sand-stone and green-
stone, as in the island of Arran.

" 3. It resists the action of the weather very



Jameson's

description.



1 This epithet seems only applied to basalt, wacken, klingstein,
ava, pumice.



16*8 DOMAIN II. SILICEOUS.

obstinately. After very long exposure it becomes
covered with a thin crust, which has usually a
greyish-white colour, but a reddish crust in iron-
shot varieties.

" 4. Like basalt it forms single conical hills ;
but they are not so regular, and are more marked
with cliffs and irregular rocky forms.

" 5. Excepting small traces of iron-pyrites, and
iron-sand, it contains no ores.

" 6. It appears from observations I have made
in the islands of Arran and Lamlash, in Dumfries-
shire, and on the porphyry-slate hills near Edin-
burgh and Haddington, that this rock passes, on
the one hand, into compact felspar and clay-stone,
and, on the other, into pitch-stone and basalt.

"7. It occurs abundantly in the islands of Arran
and Lamlash, in the frith of Clyde ; also in smaller
quantities in the upper part of Dumfries-shire,
and in the county of Selkirk. Braid hills, and
part of the Pentland hills, near Edinburgh ; the
Girleton hills at Haddington, and, according to
my pupil Dr. Ogilvy, North Berwick Law, and
Traprain or Dumpender Law, in the same county,
are composed of this rock. I suspect that the
porphyry of Cumberland, which probably occurs
among transition mountains, is also porphyry-
slate. It occurs in great abundance in Bohemia ;
also, but in less quantity, in Lusatia; in the prin-



MODE IV. FELSITE. 1()9

cipality of Fulda ; in the Rhongebirge ; at Hohen-
tinel and Hogau, in Upper Suabia; at Vicenza,
in the Euganean mountains ; on the Pic de Teyde,
in Teneriffe ; and in great abundance in South
America, as I suspect that much of the porphyry
of Humboldt will prove to be porphyry- slate."

It may likewise be observed, from Brochant's
description, that patrinite sometimes occurs in glo-
bular masses, implanted in other rocks ; and also
sometimes assumes the prismatical form, in groups
of more or less regularity.

To these remarks may be added the curious
description by Klaproth, which accompanies his
analysis.

" The schistose-porphyry is a species of stone, Kiaproth's

J J account.

which, notwithstanding that it so frequently occurs,
and even in masses forming entire mountains and
rocks, yet was doomed by a singular fate long to
continue to be disregarded, unknown, and con-
founded with other stones.

: 'The first denomination, under which it has
been admitted in oryctognostic treatises, is that of
hornschiefer (horn-slate). However, this name
does not exclusively belong to it ; for which reason
various authors denote by the same name several
different species of stones. This German deno-
mination seems to have been occasioned by the
Latin corneus Jissilis of Walkrius ; though it is



1/0 DOMAIN II. SILICEOUS.

obvious from the description which he has given
of his corneus jissilis, that he did not mean to sig-
nify by that name our schistose-porphyry, which,
as it seems, he did not know, but the hornblend-
schiefer (hornblende-slate of Kir wan).

" Other authors, as Born, Ferber, receive under
this name, sometimes different varieties of the
thonsdnefer (argillaceous slate), and sometimes
glimmerschiefer (micaceous slate).

" The first oryctologist who has awakened the
attention of naturalists to the schistose-porphyry,
and given of it an accurate description, was Char-
pentier, in his Mineralogical Geography of the
Electoral Dominions of Saxony. At the same
time he gave to it exclusively the name of horn-
slate, in which he was followed by most of the Ger-
man mineralogists. But Werner thought other-
wise. He left this name at first to that species of
stone, which afterwards has been called kiesel-
chiefer (siliceous slate); and denominated that
which is the subject of the present essay schistose-
porphyry, in order to distinguish it as a peculiar
species of porphyry. In fact, it exhibits the mi-
neralogical character of porphyry : as it princi-
pally consists of an homogeneous, hard, siliceous
and argillaceous aggregate, in which, though but
sparingly, and singly, are interspersed small la-
mellae of feldspar, besides minute grains of horn-



MODE IV. FELSITE.



blende ; yet at the same time it is distinguished
from the common species of porphyry, by its gross
slaty fracture. But since to the principal mass
of this stone the name of klingstein (sounding-
stone) has been given ? because its larger plates,
when struck, give a sound almost metallic; it
seems that the name of klingstein porphyr (kling-
stone porphyry) would be more characteristic, and
more conformable to analogy.

" From this short historical account it may
sufficiently be seen, what great uncertainty and
want of accuracy has even of late prevailed in the
geological knowledge of the mountainous part of
the globe. For this reason the editor of the ma-
gazine for the Natural History of Swisserland has
highly merited the thanks of the public, for having
occasioned, by means of a prize -question, that
this intricate subject has been investigated by two
learned mineralogists, Karsten and Voigt, and cor-
rectly explained in their two papers, crowned with
the prize : " On argillaceous schistus, horn-slate,
and on wake*."

" The klingstone-porphyry belongs to that divi-
sion of mountains which, by modern geologists, are

* " Magazin fur die Naturkunde Helvetiens," by Dr. Hopfner.
Zurich, 1788. Vol. iii. page 168, seq.



172 DOMAIN II. SILICEOUS.

classed with the trap-mountains*. In Germany
it occurs chiefly in the middle mountains of Bohe-
mia, in Upper Lusatia, and in the district of Ful-
da. It does not form coherent ranges of moun-
tains, but always only detached masses of rocks
and insulated mountains, commonly on the side
of similar basaltic mountains. It belongs to the
most durable saxa, and resists the withering in an
eminent degree. Only on its surface some decay
takes place, by which it acquires a pale clayey
crust, the smoothness of which renders the ascen-
sion on klingstone-porphyry mountains somewhat
unsafe. Some of these mountains, though but
sparingly covered with fertile soil, are nevertheless
well invested with plants and forest-trees ; as for
instance the Donnersberg, near Milleschau, and
the Schlossberg, near Toplitz. But most fre-
quently the klingstone-porphyry occurs in the
shape of cliffs, which are dentated in a grotesque
manner, from the unequal, mostly vertical, se-

* In the German frapp formation. This expression is now,
according to Emmerling, by the latest German mineralogists used
to signify all mountains constituted of griinstein, amygdaloid, schis-
tose-porphyry, basaltes, and their subordinate species. All these
are called trapp-gelirge, since those saxa not only occur in moun-
tains of the same kind, but also very much agree in their geo-
gnostic relations} so that evidently they are of the same formation.
Transl.



MODE IV. FELSITE. 173

parations into large plates and ill-formed pillars.
Examples of such grotesque rocks, are the Biliner-
stein, near Belin, and the Engelhaus-berg, not far
from Carlsbad.

" To complete the history of this stone, I must
briefly notice an opinion which has of late been in
vogue, and even now seems to have its abettors.
I mean, that the klingstone-porphyry, as jvell as
the basalt, the amygdaloid (mandelstein), and
other trap mountains, have been considered as
volcanic productions, or lavas. It does not be-
long to the object of the present inquiry to repeat
and to examine what on both sides of the question
has been argued, and sometimes with violence
disputed. I shall only mention, that in the se-
veral attentive inspections with which I examined,
in the middle mountains of Bohemia, the sites of
basalt and klingstone-porphyry, I could not dis-
cover the least vestige of a crater, or other signs
of a volcanic nature ; no more than any other un-
prejudiced observer would have been able to find.
" To these short geognostic remarks I shall
now add the description of the external characters
of the mass, which chiefly constitutes the kling-
stone-porphyry.

" The colour of the klingstone^ is grey, now and
then a little inclining to the green. It occurs
only massive. It has a fine grain, an uneven,



174 DOMAIN II. SILICEOUS.

coarse-splintery, fracture, and bursts into thick
slaty fragments. The edges of its thin splintery
fragments are transparent. It is pretty hard, and
at the same time considerably tenacious. When
triturated, it yields a light grey powder.
" Its specific gravity is 2,575.
" The lamella? of a greyish white, strongly
splendent felspar, which are interspersed in its
substance, together with the very minute crystals
of a black hornblende, give to it the character of
porphyry."*

The result of the analysis is

"Silex 57,25

Alumine .... 23,50

Lime 2,75

Oxyd of iron . . . 3,25
Oxyd of manganese 0,25

Soda 8,10

Water 3

98,10

" The reflecting natural philosopher will know,
without my suggesting it, how to appreciate the
value of this discovery of the presence of soda, as
a constituent part, in a stone which occurs in
masses of the size of entire mountains. It opens

* Klaproth Anal. Essays, ii. 182.



MODE IV. FELSITE. 175

to him a new view, and leads him a long step
farther in his geological inquiries. We now see
that there is no longer any occasion for the theory
hitherto prevailing, according to which it was ima-
gined necessary to consider all the soda, which in
nature occurs either in a free, that is uncombined,
or in the carbonated state, as an educt arising from
a decomposition of rock salt, or of sea salt, or of
that from saline springs, supposed to have been
carried on by nature, and to have taken place in
an unknown manner.

" The klingstone employed in the preceding ex-
periments was from the Donnersberg, near Mille-
schau, the highest of the middle mountains in
Bohemia. The whole mass of this majestic cone,
which is above two thousand five hundred feet
high, consists entirely of this stone. From its
summit the picturesque fields of Bohemia, extend-
ing for many miles around, present themselves to
the eye, collected as it were in a pleasing minia-
ture painting ; while at the same time, at a farther
distance on the eastern horizon, the Bohemian and
Silesian Giant-mountains, and on the west the
Franconian Fichtelgebirge (mountainous region),
are discovered.

;< If we now reflect, that in this enormous mass
of rock, the soda constitutes nearly the twelfth
part of the whole, I hope it will not be thought an






176



DOMAIN II. SILICEOUS.



exaggeration to say, that this mountain alone is ca-
pable of providing, for a long succession of years
to come, all Europe with sufficient soda; pre-
supposing, however, that expedients should be
devised to separate this alkali from the stone by a
cheap and profitable method*."



STRUCTURE III. EARTHY.

This is described by Saussure as of an earthy
or granular appearance, with long and irregular
crystals of black siderite, sometimes greenish.
The rock was mistaken for a sand-stone. He also
mentions, 1 1 36, a rock of a violet red, which he
conceives to consist of the earth of felspar not
crystallised.

Earthy ~klingstein-porphyry ? from Mont Dor,
described by Daubuisson in his account of the
basalts of Auvergne.

The following varieties may also be added from
the former great petralogist :

Rocks of felsite, with veins which at first might
be taken for granite, but upon examination is
found to present only felspar and mica, and some-
times only felspar confusedly crystallised. Sauss.
1 194.

* Ib. 193.



MODE V. GRANITE. 177

A rock of green and violet layers, being a kind
of felsite. Sauss. 1448.

What he calls a trap, with a paste of a greenish
grey felsite, and grains of quartz and felspar.
2043.

1558. The rotten-stone of England may be Rotten-stone,



Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 13 of 35)