John Pinkerton.

Petralogy. A treatise on rocks (Volume 1) online

. (page 9 of 35)
Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 9 of 35)
Font size
QR-code for this ebook


Basalton, or whin, from Salisbury Crags, near
Edinburgh.

The same, from the Malvern hills.
Basalton is common in the pavement of London,
/

STRUCTURE II. SLATY BASALTON.

Grunstein This is the green-stone slate of the Germans,

slate.

being composed of siderite and compact felspar,
or felsite, which is sometimes more abundant than
the former. It is said to form mountains in
Sweden, and abounds near the mines of Adelfors,
being often metalliferous. If the felsite generally
.exceed in quantity, it ought to be classed under
that rock.

Kiinkstein. The porphyry slate, or clink-stone porphyry, of
Werner, basalte en table of the French, seems an
intimate mixture of iron and felsite, and is often
found in basaltic countries. It has been analysed
by Klaproth, who found eight parts of soda in a
hundred. How it came to be classed among the
basaltic family can scarcely be imagined, except
from its local -situation, a circumstance too pre-
ponderant with Werner; it being as often found
in the vicinity of basaltin, as lava with a base of



MODE V. PORPHYRY.

felsite is in that of lava with a base of siderite. It
is surprising that the French mineralogists have
not adduced this circumstance in favour of the
volcanic origin of basaltin. Clink-stone however
has no sort of relation to the family of basalts, as
the chemical analysis infallibly demonstrates ; for
it only contains 3 in the 100 of iron, while all the
other modes present more than 20. It is there-
fore here classed under Felsite, with which the
analysis strictly corresponds, except that there is
double the quantity of iron, which imparts the
black colour.

MODE V. PORPHYRY.

This rock belongs to the division here called Name.
Intrites, as consisting of crystals or grains im-
bedded in a base or paste, in contradistinction
to Granites formed by simple coherence, and to
Glutenites, (both also derived from the Latin), in
which the particles are cemented together by
the same or by a different substance, scarcely
visible, or at least not so abundant as in the
Intrites*. This last denomination, besides in-
stantly recalling to memory the nature of the
rock, would prevent the misapplication of the

* In like manner the Glandulites of Saussure are those stones
\vhich include glands or kernels.



DOMAIN I. SIEEROirg.



classical term porphyry to many substances,
which have only a very faint and distant resem-
blance.

The term porphyry is therefore here restrict-
ed to its proper and peculiar sense of a base
sprinkled with crystals of felspar. The word in
the Greek implies a purple, or rather red stone ;
and in severe classical precision ought to be
confined to that colour, common among the
monuments of antiquity : but as denominations
derived from colour, the worst of all distinctions,
have been forced to be extended, the black, the
grey, the bluish, and even the green, having
the same base of trap or basaltin, must be in-
cluded. But the base being the sole ground of
the present classification, all the other kinds are
considered as Intrites, and reserved for separate
descriptions.

Base. It was long imagined that the base or ground

of porphyry consisted of jasper ; but this suppo-
sition has been finally rejected, and it has been
found to be trap, from its fusibility and other
chemical properties, and likewise from its exter-
nal attributes. Like basaltin, it presents crys-
tals of siderite, grains of quartz, and sometimes
glandules of chalcedony and of steatite, which
last perhaps forms the green matter in Swedish
porphyry. The crystals of felspar are generally



MODE V. PORPHYRY. 77



rectangular, but sometimes oval or otherwise
irregular. When they are scarcely visible to
the naked eye, the substance is here called por-
phyrin ; and when they exceed an inch in size
the term porphyron may be applied.

Genuine porphyry abounds in many parts of
the world, and often forms entire mountains*
Like siderite it has been found to alternate with
gneiss, and it occurs in a columnar form. Among
the defects of orology, and even of the Wer- Werner's
nerian theory of formations, maybe chiefly par-
ticularised the classification of the porphyries,
vaguely so called, which are arranged under one
head, whether the base be keralite, felsite, pitch-
stone, or even serpentine, or indurated clay;
while felspar, like mica, may be occasionally
found in most rocks, and these pretended por-
phyries ought all to be referred to their several
bases. The name has even been extended to
rocks with calcareous or other crystals: and as
strict definitions form the first foundation of every
science, no argument can more clearly evince
the necessity of new and abundant denomina-
tions of rocks, than this confusion of substances
of a nature wholly remote; and so frequent and
important, that no geological work can be pro-
perly understood, except the author use much
circumlocution. For to extend the term p or-



73 DOMAIN I. SIDEROUS.

phyry to every substance in which small crystals
are imbedded, is as absurd as it would be to con-
found granular limestone with granular quartz ;
or any other remote substances merely of similar
structure, or even aspect.

STRUCTURE I. PORPHYRY WITH LARGE CRYSTALS
OF FELSPAR.

Aspect 1. Red Porphyry. This kind is fre-
quent in ancient monuments. The crystals are
seldom so regular as those of the next structure.
It is sometimes interspersed with globules of a
finer porphyry, or even of porphyrin.

Porphyry, from Egypt, or the ruins of Rome.

The same, from the Grampian Mountains in
Scotland. It chiefly occurs in Glenco*.

The same, from Corsica.

Aspect 2. Black. A fine column of this kind
is in the church of St. Prassede, at Rome.

Aspect 3. Green. This has sometimes been
called verd-antique, but the proper verd-antique
is a mixture of serpentine and white marble. The
green porphyry has also been erroneously supposed

* Which must not be confounded with Glen Cro, not far from
Tnverarv.



MODE V. PORPHYRY. 79

to be one of the Ophites, or snake-stones of
Pliny.

Saussure, and innumerable others, misled me Ophite.
concerning the Ophites of Pliny. The passages are :
- Pretiosissimi quadam [marmora] generis, sicut
Lacedcemonium viride, cunctisque hilarius. Sic et
Augusteum, ac deinde Tiberianum, in JEgypto,
Augusti ac Tiberii primum principatu reperta.
Different iaque eorum est ab Ophite, cum sit illud
serpentium maculis simile, unde et nomen accepit ;
quod hcec maculas diverse modo colligunt ; Augus-
teum undatim crispum in vertices; Tiberianum
sparsa, non convoluta, canitie. Neque ex Ophite
columns nisi parvce admodum inveniuntur. Duo
ejus genera, molle candidum, nigricans durum.
xxxvi. 7. edit. Brotier, Paris 1779, 12mo.

Again, c. 22, speaking of stones used for
making mortars. Potiorem ex alabastrite JEgyptio,
vel ex Ophite albo. Est enim hoc genus Ophitis.
ex quo vasa etiam et cados faciunt.

These passages may be thus interpreted :
" Some marbles are of a very precious kind, as
the green of Lacedemon, which is also more cheer-
ful than any of the others. So also the Augustean,
and afterwards the Tiberian, first discovered in
Egypt during the reigns of Augustus and Tiberius.
The difference between these marbles and ophite
consists in this, that the latter resembles the spots



80 DOMAIN I. SIDEROUS.

of serpents, whence its name is derived ; whereas
the marbles display their spots in a different man-
ner, the Augustean being crisped into wavy tops,
while in the Tiberian the white is scattered, not
convolved. Nor can any columns be formed of
ophite, except of a very small size. There are
two kinds of it, the white being soft, the blackish
or grey hard." He then proceeds to state that
both were used to appease head-achs, and against
the wounds of serpents ; particularly a kind of
ophite named Tephria, because it was of the co-
lour of ashes; and also called Memphites, from
the place where it was found, being of a gemmose
or sparkling appearance.

The other passage implies that " good mortars
may be made of Egyptian alabastrite, or of white
ophite, for this is a kind of ophite of which they
make even vases and larger vessels."

Lucan also has,

Quam parvis tinctus maculis Thebanus ophites.
te Like Theban ophites tinged with small spots."

Pliny is not very accurate, and it is more pro-
bable that the ophite came from Thebes in Upper
Egypt, than from Memphis. It must however
have been wholly remote from green porphyry,
being probably the Thebaic stone, mentioned by
Theophrastus, of a dark colour sprinkled with



MODE V. PORPHYRY.

golden drops ; that is, as Wad explains, a dark
ollite interspersed with golden mica *. The white
was probably a spotted marble or alabaster, for
the ancients arranged stones by faint resemblances ;
but white ollite, or rather massive steatite, is not
unknown in Saxony and other countries.

Green porphyry occurs in large blocks near
Ostia, which, as Ferber observes, was the old har-
bour where the Egyptian ships unloaded f: but
all the other ships bound for Rome also arrived
there ; nor do I find any mention of green por-
phyry in the memoir concerning Egyptian geo-
logy, which M. Roziere presented to me. This
porphyry contains crystals of siderite, and fre-
quently spots of chalcedony. Ferber observed

observations.

the following varieties :

Dark green, with fair green spots ; common.
Dark green, with white spots.
Dark green, with black spots.

* Fossil. JEgypt. p. 27. This subject will be further illustrated
in treating of the Magnesian Rocks, and of the ancient marbles,
where it will he seen that the green porphyry was unknown till the
latter times of the Roman empire. In describing this substance,
Da Costa, p. 288, justly observes, that there are no Grecian nor
Roman relics of it ; and he concludes that very little was found even
in Egypt. It is now well known that it is not an Egyptian pro-
duct; but vast masses remain near the port of Ostia, where it was
probably left pn the irruptions of the barbarians in the fifth century,
when the arts were interrupted and abandoned.

f Travels in Italy, 225.

VOL. j. <*



82 DOMAIN I. SIDEROUS.

Fair green, or yellowish green, with black spots.

He also mentions a dark green porphyry, of
which the trap base sometimes passes into crystals
of siderite ; while the large white crystals of fel-
spar are so numerous, that it might be called a
white porphyry.

It is believed that green porphyry of this struc-
ture is found in Ireland, and in Cumberland, and
also in Norway.

The ancient was probably found in Thessaly,
as will appear in discussing, under one point of
view, the Ancient Marbles in Domain V.
statement Saussure mentions the following :

A porphyry with a base of greenish grey felsite,
with very small crystals of felspar, and some glp-
bules of lime; the presence of the latter in so
hard a rock being regarded as singular. 1578.

A singular porphyry of a chocolate colour, with
crystals of blue felspar. 1448.

Not far from Frejus is a mountain of red por-
phyry, with some grains of quartz. 1453.

At Esterelle, between Nice and Frejus, are
rocks of porphyry, the base being of the colour of
wine lees. 1436.

P Durance f ^ e ^* * ves ^ ie ^^ owm g account of the ppr-
phyries which he observed in pebbles, in the bed
of the river Durance, which has been long cele-
brated for its variolites.



MODE V. PORPHYRY. 83

1539. B. " Green porphyry. The cement of
this porphyry approaches likewise to that of the
ophite *; its colour is however less beautiful : it
is a green which verges to the deep grey : it like-
wise assumes a surface less uniform and less soft
to the touch. In other respects its fracture and
hardness are the same, but it is a little more re-
fractory, and the glass it yields is less hard and
opake. The small fragments of this glass are
however attracted by the magnet.

" The crystals of felspar wilich this porphyry
contains are, as in the ophite, lengthened oblique
angled prisms, of a white inclining a little to green;
of an unctuous and milky lustre ; their fracture is
more compact, and presents thicker laminae than
common felspar.

1539. C. " Red porphyry. The cement of this
porphyry is of what I call primitive petrosilex.
In the rolled pebbles its surface is pretty uniform,
almost soft to the touch. It breaks into irregular
fragments in sharpish angles, almost opake on
their edges. Its fracture is scaly with very thin
scales, which, viewed by a microscope, appear
semi-transparent and whitish, whilst the base is
of a pretty deep wine red.



* By ophite, Saussure, like many, others, erroneously understands
the green porphyry.



84 DOMAIN I. SIDEROUS.

" This cement is more than semi-hard, it easily
gives sparks under steel, and may nevertheless be
scratched by a knife, the streak being of a rose
colour. It melts with difficulty under the blow-
pipe into a semi-transparent glass, grey and full
of bubbles, mixed with some brown dots, which
are attractable by the magnet.

" The grains are of felspar, white, yellowish,
rarely crystallised with regularity, and of the unc-
tuous nature of the preceding.

1539. D. " Black porphyry. The paste of this
is of a fine deep black, approaching a little to
blue : its exterior surface is pretty uniform, and
almost soft to the touch. Its fracture delicately
scaly, as that of the preceding ; but its hardness
rather less, although it yields some sparks. It is
still more refractory ; the flame of the blow-pipe only
whitens and blunts it a little on the thinnest edges.

" The grains, of a greenish white, have no regu-
larity ; they are cemented in the black base of the
stone, in all sorts of forms. Their fracture is
most frequently scaly : there are however to be
seen some marks of the laminar texture of the
felspar, and it is also, as in the others, of the unc-
tuous kind.

1539. E. " Brown porphyry. Its cement is
brown, rough, and of an earthy aspect ; it is how-
ever pretty hard. The grains, seldom regular, are



MODE V. PORPHYRY. 85

of an unctuous felspar, a little compact, and of a
greenish tint*.

1539. F. " Grey porphyry with a cement of
petrosilex, of a greenish grey, enclosing a number
of crystals of unctuous felspar of the same colour,
though a little whiter, some pyrites, and some
black ferruginous spots.

1539. G. " Schistose porphyry with a blackish
cement the colour of iron, with a scaly and bril-
liant fracture, hard, containing crystals of dry
white felspar, opake, which bubbles and readily
melts under the blow-pipe, and other crystals of
hornblende, pretty hard, of a blackish green."

Add porphyry with black chalcedony, from
Chemnitz, in Saxony. (Linn, a Gmelin,



Aspect 4. Blue. Dark indigo blue porphyry,
with crystals of yellowish felspar, from the isle of
Rasay, Scotland.

See Mr. Jameson's Mineralogy of the Scotish
Isles, ii. 117. He says the base is betwixt clay
and hornstone, so it is only placed here to excite
further enquiry concerning so beautiful and un-
common a rock.

* Verd d'ceillet. The French of Swiss and German authors is
often peculiar.



86" DOMAIN I. SIDEIIOUS.



STRUCTURE II. WITH SMALLER CRYSTALS.

Aspect 1. Red. Of Egypt, the felspar being
frequently in white or in flesh-coloured prisms*.

The same, with white only, the porphyry of
Pliny. It is sometimes interspersed with masses
of a lighter or darker colour.

The same, from Corsica, from Lesterelle in
Provence, Scotland, &c.

The same, with crystals of siderite.

Aspect 2. Brown. Of this Ferber mentions
two varieties; the liver brown with light green
spots, and the dark brown with spots half black
and half green ; perhaps he means crystals of
black siderite and green felspar.

Dark brown porphyry, speckled with numerous
small crystals of felspar, and others of siderite
and quartz, with reddish and green nodules, from
Sweden, where it is manufactured and takes a
high polish.

Aspect 3. Black. Entirely resembling the red,
except in colour. Of this there are two large
columns in a church near the gate of St. Paul, at
Rome.

* Wad, 12.



MODE VI. PORFHYRIN.

Aspect 4. Green. Resembling the red.

The same, with crystals of siderite.

From Ferber's description it would seem that
the felspar is wanting, in which case it is a trap
or basaltin.

The same, with small crystals of felspar in
white spots, commonly irregular, and twisted like
worms; the Porjido yerdejiorito*.

The red also occurs in Egyptian monuments, as
well as the black f . Green porphyry is also found
in Corsica and Norway.



MODE VI. PORPHYRIN.

This name has been adopted for porphy-
ritic substances, in which the crystals of fel-
spar are so small as almost to escape the eye,
or not be discoverable without a lens. But
somewhat of th regularity of true porphyry
must be observable, otherwise the substance
must be considered merely as a mingled basaltin.
On the other hand, the mixture of a few grains
of quartz may be admitted in a porphyrin ; but

* One kind so called is not a porphyry, but a waved mixture of
siderite and felspar, as if daubed with a brush. See Anomalous
Rocks.

t Wad, 12, 13.



88 DOMAIN I. SIDEROUS.

if the base assume the granitic form, it must be
regarded as a granitic porphyry.

porphyry. The Swedish porphyry, already mentioned,
approaches nearer to a porphyrin ; specimens
of which are common in most porphyritic re-
gions, forming the passage from basaltin to por-
phyry.



MODE VII. PORPHYRON.

When the crystals of felspar exceed an inch
in length, and are distant from each other, cir-
cumstances which occur in the large scale of
nature, the rock may be termed Porphyron.
The utility of these divisions will be more fully
understood, as the science becomes, more and
more studied.



MODE VIII. PORPHYROID.

This denomination includes such substances
as approach the porphyritic structure. In a
strict derivation of the term porphyry, as al-
ready explained, the black and green kinds
could only be termed porphyroids; but as this
severity would too much violate common usage,



MOBZ IX. AMYGDALITE.

the term porphyroid, as admitted into the side-
rous division, must be restricted to such rocks as
have a base of siderite, basalt, or basaltin, pre-
senting an appearance of porphyry. Some of
the primitive grunsteins of Werner fall under
this distribution. When the base is siderous,
but the square crystals are barytic or calcareous,
&c. this denomination may also be adopted*.
The mixtures called granitic porphyroids, &c.
are to be arranged under their proper domains.



MODE IX. AMYGDALITE.

Where the distinctive characters of a sub-
stance vary much, they are omitted, to avoid
unnecessary prolixity, especially as they may
be found in the common books of mineralogy ;
and rocks should be studied in themselves, as
well as in books, for the only use of any classi-
fication is to assist the memory.

This substance, the mandelslein, or almond-
stone of the Germans, has a base of coarse trap
or basalton, in general black or brown, inter-
spersed with nodules or kernels of chalcedony,
agate, calcareous spar, zeolite, and green mag- Agates.

* The green siderite, with crystals of calcareous spar, (Sauss.
1. 13Q, 4to.) may belong to this division.



90 DOMAIN I. SIDEROUS.

nesite, or magnesian earth mingled with iron*.
The agates afford valuable materials for manu-
facturers ; and the rock abounds in many coun-
tries, as at Oberstein on the Rhine, Kinnoul and
other places on the river Tay, in Scotland,
whence the English lapidaries have called the
latter agates Scotch pebbles f. In the north of
Italy the same rock presents chalcedonies, which
are sometimes enhydrous, or contain a drop of
water. In the Faroe isles the chalcedony com-
monly assumes the stalactitic form; and, as
Landt observes J, it has been found modelling
itself on. straw or moss, whence it clearly appears
to have been deposited by water; either heated
by its own caloric (for if water contained no
principle of heat it would become ice), or by
subterraneous fires, as the fountain of Geyser in
Iceland deposits silicious concretions.
Formations. Werner considers Amygdalite as of two form-
ations ; the Transitive, the base of which he calls
wacken, an argillaceous rock, sometimes inclin-
ing to basaltin, which it generally accompanies,
and sometimes to iron-stone, a mixture of iron



* This may also be called a lole, a shorter word than lithomarga,
and expressive of the same substance, as appears from the analyses.

f This name seems also a distinction from the English pebbles
in pudding-stone, &c. some of which are as beautiful as agates.

J P. 146.



MODE IX. AMYGDALITE.



and clay, which is also the chief repository of
prehnite. The other formation belongs to his
Floetz, horizontal, or stratiform rocks ; and he
also describes the base of this as being wacken,
or rather decomposed grunstein, which, accord-
ing to his theory, generally lies under basaltin
and above clay. But Mr. Jameson, to whom
we are greatly indebted for an exposition of the
Wernerian system, omits amygdalite in his de-
scription of wacken ; and Brochant regards the
base of amygdalite as a decomposed siderite or
grunstein, and it certainly belongs to this do-
main. It is believed that olivine, though fre-
quent in basaltin, has never been observed in
amygdalite, in which the silicious parts assume
a different form.

Some French mineralogists have supposed Origin,
amygdalite to be of volcanic origin ; but Patrin,
though an ardent volcanist, has rejected this
idea, arid arranges it after porphyry, as he ob-
serves that the base is sometimes siderite, some-
times trap. The cavities are also larger than
any found in lava; and though agates be so
named from the river Achates in Sicily (in the
south of that country, and at a great distance
from Etna), it appears not that agates have ever
been observed in any volcanic region.

Amygdalite, like basalt, often contains no-



DOMAIN I. SIDEROUS.



dules of common steatite, and small crystals of
siderite. As it only takes a very coarse polish,
the base is properly a basalton.

STRUCTURE I. AMYGDALITE WITH AGATES.

From O her stein on the Rhine.

The same, with cubic zeolite.

The same, with veins of glassy chalcedony, ac-
companied with a band of the colouring matter,
which would form agate.

Brown amygdalite, from the same.

Amygdalite, with chalcedony, zeolite, c. from
the Isle of Skey.

Faujas has given a good list of the products of
Kinnoui. Kinnoul, but ridiculously calls them lavas. He
mentions black basal tin joined to basal ton, the
latter presenting small crystals of felspar, so as to
assume a porphyritic appearance. The same com-
pact basaltin, in columns. Green basalt, very
firm and sonorous. Basalt with crystals of fel-
spar, and attracting the magnet. A square prism
of the same, with a carnelian on one of the sides.
The same, with globules of green earth, agate, and
calcareous spar, &c.

Moca. Beautiful agates, or what are called moca-

stones, also occur in a rivulet called May, which
falls into the Ern near the house of Condie, in



MODE IX. AMYGDALITE.

Perthshire. Moca-stones are also said to be
found on the banks of the Tweed, being chalce-
dony mingled with green earth, bitumen, &c. in
the form of moss, and other appearances. They
receive their name from Moca, in Arabia ; that is,
like many other substances, not from their native
site, but from the mart where they were sold,
being brought to Moca from Cambaya in Hin-
dostan, which also transmits beautiful carnelians
and chalcedonies*. There are probably rocks of
amygdalite in that vicinity. It would appear
however that this rock is among the rarest pro-
ducts of nature, having seemingly been observed
only in the four countries above mentioned ; Hin-
dostan, Sicily, Scotland, and the neighbourhood
of Ober stein.



STRUCTURE II. AMYGDALITE WITH CALCAREOUS SPAR.

Of this the toad-stone of Derbyshire affords Toad-stone,
a well-known example. Patrin mentions another

* It is said that the fine carnelian is produced by art from no-
dules of a kind of chalcedonic flint, which are left in the heat of
.horse-dung for many months, One of these flints I received from
my highly-respected friend Mr. Ferguson, whose noble collection of
minerals is known to all Europe. But, in the oriental phrase, his
love of science, and generous spirit, surpass all the gems in his
cabinet.



94 DOMAIN I. SIDEROUS.

from Strelka, in Siberia, with crystals of felspar
and globules of steatite, in a base of liver-coloured



Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 9 of 35)