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the slight effervescence that was observed, be in-
filtrated, and deposed as a foreign body in its ex-
ternal pores.

" I have seen in Italy antique works, which
were said to be basalt, but which appeared to me
of a kind of rock very similar to this, and conse-
quently very different from real volcanic basalts.
A statue of a child, that is shown in the gallery
of Florence under the name of Britannicus, and
which is said to be of basalt, is most likely of this
same kind of rock. I have had a piece of this
sand-stone worked ; and the kind of polish which
it has taken, perfectly resembles that of this sta-

sembles the green bricia of Egypt, as I am informed by an ingenious
French traveller.
* Sausa. 493.




A HIS earth is obtained in the state of
greatest purity from alum, which is a mix-
ture of argil and sulphuric acid. If it con-
tain oxyd of iron, as is frequently the case,
it emits a particular smell, when breathed
upon, well known by the name of an
earthv smelL


With heat it loses its water, and dimi-
nishes in bulk ; but a very violent heat
converts it into a white amel. When com-
bined with lime it easily enters into fusion.

Argil, also called Alumina by recent
chemists, 4s of great utility, as forming the
basis of many manufactures, such as brick,
porcelain, and earthenware. It constitutes
98 parts in the 100 of corindon; under
which division are now classed the most
perfect of the precious stones, after the
diamond, such as the sapphire, ruby, and
oriental topaz. It is hence not only one
of the most noble, but one of the most
useful of the earths ; loam or fertile soil
being a mixture of about 30 parts argil
with 70 of fine sand ; while mould chiefly
consists of animal and vegetable remains.

In the primitive rocks argil is an im-
portant feature, forming about a fifth part
of felspar, and a third of mica. The most
ancient slates abound in argil. It is often
so homogeneous that it cannot be regarded
as the waste of former mountains, but a
pure deposit of primeval waters. In the


primitive schisti however there is still a
great preponderance of sand; and the
glossy appearance may sometimes proceed
from decomposed mica.

The argillaceous rocks are mostly of a
simple and uniform appearance, and do
not admit the numerous modifications of
some other substances. This earth is chiefly
eminent in gemmology, where it consti-
tutes some of the most beautiful varieties.
The argillaceous rocks are never crystal-
lised, and present but small splendour in
their appearance*; hence they are very
seldom used in the ornamental arts, and
are chiefly important in a geological point
of view, where they often rank among the
most important primitive substances. Yet
even in this light they have not been
treated with the attention and minute in-
vestigation which have been bestowed upon
the Siliceous and Calcareous Divisions.

The essential part of the argillaceous

* Brongniart, i. 512, informs us, that as the argils never crystal-
lise, they aflford no species, A further proof that this term is foreign
to mineralogy.



rocks being alum, it seems the most na-
tural progress to begin with those sub-
stances which chiefly supply commerce
with that earth.


Of this there are two very different structures;
the alum rock of Tolfa, which yields what is
called the Roman alum, and the common alumi-
nous slate.
Alum of It has been said by some that the rock alum

Rocca. J

of the middle ages derived its name from a town
in Syria, called Roch or Roque, Rocca ; but a
pilgrim having observed the same kind of rocks
near Civita Vecchia, the Pope founded the ce-
lebrated manufactory which supplied Europe
lor some time*. The description of the latter
has been given by several mineralogic authors
under the class of salts ; but it may be interest-
ing to present the accurate account of Ferber,
who mentions, that the rocks which yield the
Roman alum constitute white, high, and argil-
laceous hills, of a compact structure, and with

* Wall. ii. p. 43. Alum is classed among the salts by chemical
writers, and is called sulphate of argil.


scarcely any visible horizontal beds ; but there
are some fissures filled with quartz, yielding
what are called Tolfa diamonds*. He after-
wards proceeds:

" The alum hills are very high, shining, white Tolfa.
rocks, separated by a long valley, and large ex-
cavations, which are made in the following man-
ner. The workmen descend by ropes to the
steep rocks; thus suspended, they bore blasting-
holes, fill them with cartridges, free the rocks
which by former blastings are loosened, and
then are pulled up again. The firing of the
powder is done by dry branches and leaves,
which experience has taught them to throw
from on high to any place below.

" The alum rock is whitish grey, or chalk-
white 5 extremely compact, and remarkably
hard. Scraped with a knife it yields an argil-
laceous powder, which does not ferment with
any acid, as it is penetrated by the vitriolic
acid, and composed of an argillaceous substance.
There are some bluish grey shivery pieces, which
are rejected as unfit, and probably are the re-
mains of the natural argillaceous stone, before it
was sufficiently imbibed and whitened by the
vitriolic acid. In some cracks appears a chalk-

* Italy, i'06.


white ductile clay. Some pieces are bluish
grey, with white spots, produced by the acid.
They much resemble the half-dissolved black
lava in Solfdtefra, with white, garnet-like,
schorls ; with this difference, that in Solfaterra
the subterraneous acid worked upon lava, and
here upon an argillaceous bluish stone. The
acid seems in this place likewise to be produced
by subterraneous steams, which, penetrating the
argillaceous stones, changed them into alum
ore. I could not ascertain whether there be
near Tolfa ancient volcanoes ; but I saw lava-
fragments in the wall under the boiling-pans,
and therefore they cannot be far distant.

" By all this it appears that the aluminous
rock at Tolfa is an indurated clay, having im-
bibed and been whitened by a vitriolic acid, and
contains some few calcareous particles, which,
in the alum manufactories, precipitate in the
wooden rills or troughs, under the form of sele-
nites. It is a compact and sound rock, neither
stratified nor shivery and slaty. Some nearly
perpendicular white-grey quartz veins, three or
four inches wide, cross it from top to bottom;
and in some places appears in the midst of the
white rock a red mixture, as it were, of a colco-
thar vitrioli, or crocus mart is 9 or spotted pieces,
which resemble red and white marbled soap.


" The blasted stones are calcined in furnaces,
which have an inverted conical form. They are
in the open fields close together, surrounded and
separated by a covering of turf and mould. The
upper diameter is about eight feet. They are
filled at the bottom with wood, and then heaped
with alum-stone, which appears above the fur-
naces as an accumulated cone, nine or ten feet
high, which is nearly answering to the :.epth of
the furnace. Then fire is set to the wood by a
square vent near the bottom, and the whole is
burnt down in about three hours' time; which
is, as they told me, the requisite time for burn-
ing: after which the heated stones are carried
to the boiling-house, distant about one Italian
mile from the quarries. Here they are put into
large pits, or square wooden reservoirs, half sunk
into the ground ; where they are steeped in a
convenient quantity of water, which, after suffi-
cient dissolution of the alum, is by troughs con-
veyed into the alum-house, and in large square
wooden settlers, that the dregs may settle at the
bottom. This done, the clear lixivium is poured
into brass pans, and, after sufficient boiling, con-
veyed into wooden coolers, on whose sides the
alum crystallises white and reddish. Before the
inspissated brine be conveyed into the cooler,
they stop it for some time in the troughs, in,


order to facilitate the precipitation of a reddish
selenite; and in the boiling they mix in the
liquor some lime and urine*.

" The supports of the pans are made of a grey
lava, with large white crystalline schorl-prisms,
whose quantity exceeds the mass of the ferru-
minating lava. It is found, as they told me, in
large loose pieces, at nine or ten miles' distance
from Tolfa; and it resembles much the lava of
a volcanic hill called St. Fiora, in Tuscany,
which I have seen, and shall describe in my fol-
lowing letters.

" The Tolfa alum-mines are said to have been
discovered in former times by a man, who, hav-
ing been long time a slave in Turkey, and worked
there in some alum works, guessed by the ilex
aquifolh/m, common about Tolfa, that there
must be alum in the neighbourhood. But this
shrub is found in many places where no alum is

It is evident from this account that the alum
rocks of Tolfa are very different from aluminous
slate, which shall be afterwards described.

* "If this be the case, the selcnite is in no respect a substantial
part of the Tolfa alum-stone, as the author seems inclined to sup-
pose." Raspt the translator, whose language is far from pure.



Texture, granular; on a large scale somewhat Characters,

Hardness, gypsic. Fragture, earthy or uneven.
Fragments, amorphous, not sharp.

Weight, carbonose.

Lustre, dull. Opake.

Colour, greyish white, greyish yellow, or yel-
lowish white. Bergman found it to contain about
43 of sulphur and other volatile matter ; 35 argil ;
22 silex ; and some iron.

Mr. Kirwan says, that veins of this kind have
been discovered in Bretagne; but when he sup-
poses that the Roman alum also runs in veins, he
contradicts the ample account of Ferber, no in-
accurate observer, who formally and repeatedly
informs us that it is extracted from a rock consti-
tuting hills, and containing veins of quartz *.


This substance abounds in many countries, characters.
Werner divides it into the common and the

* The name Roman alum is now also given to the finest,
wherever fabricated.


Texture, schistose.

Hardness, gypsic. Fracture, sometimes straight,
sometimes waved. Fragments, laminar.

Weight, from carbonose to granitose.

Lustre, glimmering ; the glossy kind glistening.

Colour, greyish or bluish black. It is the black
slate celebrated by the vulgar for its medical pro-

Aspect 1. Common. This is generally used in
the British manufactories of alum. The finest
specimens are from an old coal-mine near Glas-
gow, in Scotland.

Alum slate, from the vicinity of Glasgow.

The same, with some small appearances of the

The same, more expanded or decomposed, with
beautiful fibres of alum, like amianthus.

The bituminous shale of Kirwan*, though he
ranks it with alum slate, seems to belong to a dif-
ferent mode. Pyrites sometimes decompose to
alum, vitriolic acid being formed by the oxydised
sulphur, which, by exposure to air and moisture,
slowly re-acts on the argil, and forms alum.

* H. 19.


Aspect 2. Glossy. This, as already mention-
ed, has rather a metallic appearance, and is some-
times tarnished like peacock coal. In the north
of France it is sometimes found singularly hard
and compact.

Aspect 3. Alum earth. This is found com-
pact, and of a brownish black colour.


This must not be confounded with the argil- Distinctions,
laceous schistus of Kirwan, which is here called
slate, and assigned to the Siderous Domain ;
while clay slate is the schistose clay of Kirwan,
which he also calls shale, and which is often
found over coal, bearing vegetable impressions.

The argillaceous schistus, or argillite, of Kir-
wan, is the thonfchieffer of Werner; while, by
too nice a distinction, his schiejfferthon, the slate
clay of Kirwan, is our clay slate. Brochant
terms it argile schisteuse. It is less hard and
weighty than siderous slate, adheres to the
tongue, and softens in water. But all their de-
scriptions chiefly refer to that kind which is
found in coal-mines; while the most important
division is that which forms entire mountains, as


among the Andes in South America,, and in
many other metallic regions. This is in general
very far removed from siderous slate, which is
commonly quarried as valuable in architecture ;
being less ferruginous, and far more coarsely
schistose, so as sometimes to be even confounded
with grauwack. The necessity of new denomi-
nations in mineralogy is also apparent from this
example ; for while we are told by Mr. Kirwan
that the Andes chiefly consist of primeval blue
argillite, one would expect an universal reposi-
tory of slates for architecture; while in fact none
such appear, and the substance is a coarse clay
slate, slightly impregnated with iron. In like
manner Mr. Jameson must mean the present
substance when he gives us the following in-

" Clay slate is one of the most metalliferous
of the primitive rocks. It contains many of the
venigenous formations that occur in the preced-
ing primitive rocks, as tin, lead, cobalt, and
silver. Very considerable metalliferous beds also
frequently occur, and these contain copper py-
rites, red copper ore, copper green, copper
azure, malachite, iron pyrites, magnetic pyrites,
glance cobalt, grey cobalt ore, arsenic pyrites,
blend and lead glance. Gold also occurs in this
formation, and it is said also cinnabar.


" It is a very widely- extended rock. In this
country it skirts the Highlands, from Lochlo-
mond, by Callender, Comrie, and Dunkeld ; in
the whole of that extensive district resting on,
and gradually passing into, mica slate : the same
appearances are to be observed in many other
quarters in Scotland. On the Continent of
Europe it has been traced through a great extent
of country : thus it occurs in Saxony, Bohemia,
Silesia, Franconia, Bavaria, the Alps of Swis-
serland, Austria, Hungary, and many other
parts in Europe. It occurs also in considerable
quantity in North America, as Pennsylvania ;
also in immense quantity in South America:
thus it is said that nearly the whole country
between Potosi and Lima is composed of it."*

It is self-evident that a wide distinction should
be made between this important and universally
diffused substance, and the siderous slate which
is used in architecture.

In his mineralogy, Mr. Kirwan seems to have
blended the primary and secondary argillaceous
schistus, when he mentions that it sometimes
bears impressions of vegetables and shel'sf; but
in his geological essays, which are valuable as

* Geognosy, 125.

f In the vale of Chamouni it is found impressed with am-





they present a mass of information, compiled
with great labour from German authors little
known in this country, he has distinguished
them, by the divisions of his work, into primi-
tive and secondary rocks. His account of the
primitive clay slate is as follows :

" It forms whole mountains, Voigt Prack. 38.
But more commonly only partially enters into
them, as in Saxony, Charp. 175. Or entire
strata, as at Zillerthal, in Tyrol. Its mountains
are of gentle ascent.

" There is no doubt of its being often primi-
tive, for in Saxony it frequently alternates with
gneiss and schistose mica. 3 H el vet. Mag. 190.
1 Berg. Jour. 1792. 536. And with primitive
lime-stone. 8 Sauss. 144. And in Hanover gra-
nular lime-stone is found betwixt its layers.
1 Berg. Jour. 1791. 306. We have also seen
that both granite and gneiss often rest upon it.
Both Karsten, 3 Helvet. Mag. and Monnet, in
25 Roz. 85. sufficiently establish this distinction.
There are two sorts of it particularly to be
attended to, the harder and the sefter ; the
harder border upon, and often pass into, sili-
ceous schistus, or basanite, or hornblende slate.
The softer border upon, or pass into, trap, or
wacken, or rubble stone, or rubble slate, or co-
ticular slate, or indurated clay, and the harder


often graduate into the softer. 3 Nev. Nord.
Beytr. 1 69. Or border upon the muriatic genus,
and pass into schistose chlorite, or schistose talc,
or gneiss, or schistose mica. It often contains
quartz, both in veins and betwixt its laminae.
Voigt Prack. 41. More rarely felspar, schorl,
garnets or hornblende, and granular lime-stone.
Berg. Kal. 205, 206. The softer sorts are re-
markably metalliferous. Berg. Kal. Voigt Prack.
40. The famous mountains of Potosi consist of
it chiefly. 1 Berg. Jour. 1792. 545. In Saxony
it is found in primitive lime-stone. 2 Berg. Jour.
179*2, 134; and often mixed with it, as in Leske,
G. 328. It is so much the more siliciferous as it
approaches more to granitic mountains. Lasius,
121. It passes into rubble stone. 2 Berg. Jour.
1788. 493. In the argillites of the Pyrenees no
organic remains are to be found. Descrip. Py-
ren. 27- Saussure found it in the snowy regions
of Mont Blanc. 7 Sauss. 256."*

Of the secondary argillite, or clay slate, Mr.
Kirwan gives the following description :

" There can be no doubt but argillite is fre- Secondary.
quently of secondary origin ; Ferber acknow-
ledges it to be partly primeval, and partly se-
condary. 4 N. Act. Petropol. 289. Gruner

* Geol. Ess. 183.


found ammonites in the argillite near Meyrin-
gen, in Swisserland. 3 Helv. Mag. 191. In a
specimen from Hessia, mytilites occur: see
Leske, G. 339. Voigt found a lime-stone, with
petrifactions, between strata of argillite. 1 Mi-
neral. Abhandl. 86, 8?, 88. It often contains
piscine remains betwixt its laminae. Lasius, 105.
Saussure found argillitic strata intermixed with
black marble. 1 Sauss. 401. In the Hartz, im-
pressions of reeds, rushes, and pectinites, are
found on it where it adjoins to rubble stone.
Lasius, 103. 105. Sometimes it hardens, and
grows more siliceous, from the bottom upwards.
Lasius, 105. Sometimes it is harder at greater
than lesser depths. Idem, 102. In the Hartz it
alternates with, and sometimes is intimately
mixed with, rubble stone. Lasius, 138. It also
passes into sand- stone. Idem, 105. At Kinne-
culla it alternates with aluminous slate and
marlite. 29 Schwed. Abhandl. 26." *

In the account of siderous slate it has been
observed that it contains from 10 to 20 of iron.
Dr. Townson has given an analysis of argilla-
ceous schistus, or clay slate, being argil 25,
silex 60, magnesia 9, iron 6, and some petro*
leumf. The last is accidental, and he perhaps

* Geol. Ess. 241.

f Philosophy of Mineralogy^ p. 57 '


means shale incumbent on coal; but from 4 to
8 of iron may be considered as commonly be-
longing to clay slate, while the siderous or com-
mon slate, eminently so called, contains from 10
to 20.

Ferber's primitive slate is argillaceous, with
particles of mica, and crossed by veins of quartz,
which more rarely happens in the siderous
kind; and it often appears in undulating strata.
He adds, that in the Vicentine and Veronese
territories it is regarded as the deepest rock, any
subjacent granite not having been discovered.
It contains as usual metallic veins, which often
run between it and the incumbent lime-stone*.
Patrin has little enlarged on clay slate, though
a rock of the first importance; but indulges his
imagination, that the vast beds of clay have
been produced by muddy eruptions of sub-
marine volcanoes.

The fine stone used for sharpening razors, Hone,
called a hone, is commonly a clay-slate, con-
taining, like the others, about 60 parts of fine
silex. It is often of the cameo kind, or disposed
in layers of different colours, the upper of a
whitish yellow, and the under of a reddish grey;

* Italy, 37. Da Costa, p. 165, says, the black slate of Claris, in
Swisserland, which rises in slabs, contains impressions of plants
and fish ; and is of course secondary.


the first being of a finer grain, while the latter
seems to graduate into the stone used for sharp-
ening scythes, and which, from the coarser
grains of silex, becomes an argillaceous sand-
stone. Patrin informs us that hones are found
in the mountains of Jura, and the Vosges*; and
the substance was found on digging a well an
extreme depth, at Hampstead, near London.
Chinese Clay slate has seldom been used for orna-


mental purposes ; but the Chinese, a most intel-
ligent and ingenious people, and amounting, by
the most moderate computation, to about two
hundred and thirty millions of souls, or one- third
of the human race, have rivalled the ancients in
converting to utility and ornament numerous
articles of the mineral kingdom; and, among
the rest, this substance has not escaped their

" The cameo slate of the Chinese is also a
primitive argillaceous schistus, of a very fine
paste, softer than the hone, and which presents
three, or even four, successive layers, very thin,
of different colours very neatly divided, and
strongly adhering to each other.

" The Chinese artists have availed themselves
of the disposition of this stone. They form

* i. 123,


basso-relievos or cameos of it, of most exquisite
workmanship, and sometimes of considerable
size. I saw a picture made of it in the Imperial
Cabinet of Petersburg, more than two feet in
length, representing a landscape, with figures of
men and animals. These objects were of three
different colours, white, green, and red; the
ground, of a coffee colour, made the fourth.
Beautiful specimens of the Chinese cameos are
found in several cabinets at Paris, and especially
in the collection belonging to the Council of

A fine piece of the same kind appears in the
grand collection of M. Dedree, brother-in-law of
Dolomieu. Some may also exist in England;
but although we carry on the chief trade with
China, there are not so many singular Chinese
articles in London as in Paris. The Chinese
musical balls, for example, are not known in
London. They are used by the Mandarins
when inclined to sleep, the mere heat of the
hand producing various sounds, like those of the
harp of Eolus. Faujas had one dissected, when
it was found to consist of minute wires of steel,
of various sizes, disposed according to some arti-
ficial rules. The first Parisian artists acknow-

* Patrin Min. i. 124.


ledged their inability to produce such a singular

Antique. Clay slate was also occasionally used in the
arts by the ancients, for Wad, in his catalogue
of the Borgian Museum, has mentioned a frag*
ment of a small statue of a bluish grey slate, the
surface being white from decomposition. There
are also heads of battle-axes, of a grey clay slate,
veined with a deeper colour, probably from some
island in the South Sea.

This substance is often singularly contorted
in various fantastic forms, both on a large and
on a small scale.

Gmelin supposes that the softer clays arise
from the decomposition of the harder ; and he
says that rock clay is sometimes used in build-
ing*. Dr. Buchanan, in his travels in the south
of Hindostan, observed a kind of clay, which,
when dug up and dried, becomes as solid as
brick, whence he has not improperly called it

Laterite. laterite.

The materials concerning clay rock and clay
slate are unusually scanty, not only because
they are seldom used in the arts, but because
even geologists have paid far more attention to
the granitic and calcareous rocks than to the

Linn. 13;,


argillaceous, which are however of vast extent
and great importance.

The most authentic and scientific account of
the construction of that vast chain of mountains
the Andes, is contained in the travels of Helms,
a German mine-master, who was appointed to
introduce Born's method of amalgamation into
the Peruvian metallurgy. He remained in that
country from 1789 to 1793, and in 1798 pub-
lished his Journal, containing his daily observa-
tions made on the spot. In the English trans-
lation, or rather abridgement, the translator,
unaware of their consequence, has omitted many

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