from the decomposition of some of the silicates in the lava
itself--percolated through the rock, and deposited a siliceous
coating on the interior of the vapour-vesicles. Variations in
the character of the solution, or in the conditions of deposit,
may have caused corresponding variation in the successive
layers, so that bands . of chalcedony often alternate with
layers of crystalline quartz, and occasionally of opaline
silica. By movement of the lava, when originally viscous,
the vesicles were in many cases drawn out and compressed,
whence the mineral matter with which they became filled
assumed an elongated form, having the longer axis in the
direction in which the magma flowed. From the fact that
these kernels are more or less almond-shaped they are called
amygdales, whilst the rock which encloses them is known as an
amygdaloid. Several vapour-vesicles may unite while the
rock is viscous, and thus form a large cavity which may
become the home of an agate of exceptional size; thus a
Brazilian geode, lined with amethyst, of the weight of 35
tons, was exhibited at the Dusseldorf Exhibition of 1902.
The first deposit on the wall of a cavity, forming the
``skin'' of the agate, is generally a dark greenish mineral
substance, like celadonite, delessite or ``green earth,''
which are hydrous silicates rich in iron, derived probably
from the decomposition of the augite in the mother-rock.,
This green silicate may give rise by alteration to a brown
oxide of iron (limonite), producing a rusty appearance on
the outside of the agate-nodule. The outer surface of an
agate, freed from its matrix, is often pitted and rough,
apparently in consequence of the removal of the original
coating. The first layer spread over the wall of the cavity
has been called the ``priming,'' and upon this basis zeolitic
minerals may be deposited, as was pointed out by Dr M. F.
Heddle. Chalcedony is generally one of the earlier deposits
and crystallized quartz one of later formation. Tubular
channels, usually choked with siliceous deposits, are often
visible in sections of agate, and were formerly regarded,
especially by L. von Buch and J. Noggerath, as inlets of
infiltration, by which the siliceous solutions gained access
to the interior of the amygdaloidal cavity. It seems likely,
however, that the solution transuded through the walls
generally, penetrating the chalcedonic layers, as Heddle
maintained, by osmotic action. Much of the chalcedony in an
agate is known, from the method of artificially staining the
stone, to be readily permeable. It was argued by E. Reusch
that the cavities were alternately filled and emptied by means
of intermittent hot springs carrying silica; while G. Lange, of
Idar, suggested that the tension of the confined steam might
pierce an outlet through some weak point in the coating of
gelatinous silica, deposited on the walls, so that the tubes
would be channels of egress rather than of ingress--a view
supported by Heddle, who described them as ``tubes of escape.''
It sometimes happens that horizontal deposits, or strata usually
opaline in character, are formed on the floor of a cavity
after the walls have been lined with successive layers of
chalcedony. Many agates are hollow, since deposition has
not proceeded far enough to fill the cavity, and in such
cases the last deposit commonly consists of quartz, often
amethystine, having the apices of the crystals directed towards
the free space, so as to form a crystal-lined cavity or geode.
When the deposits in an agate have been formed on a crop of
crystals, or on a rugose base, the cross-section presents a
zigzag pattern, rather like the plan of a fortress with salient
and retiring angles, whence the stone is termed fortification
agate. If the section shows concentric circles, due either to
stalactitic growth or to deposition in the form of bosses and
beads on the floor, the stone is known as ring agate or eye
agate. A Mexican agate, showing only a single eye, has
received the name of ``cyclops.'' Included matter of a green
colour, like fragments of ``green earth,'' embedded in
the chalcedony and disposed in filaments and other forms
suggestive of vegetable growth, gives rise to moss agate.
These inorganic enclosures in the agate have been sometimes
described, even after microscopic examination, as true vegetable
structures. Dendritic markings of black or brown colour, due
to infiltration of oxides of manganese and iron, produce the
variety of agate known as Mocha stone. Agates of exceptional
beauty often pass in trade under the name of Oriental agate.
Certain stones, when examined in thin sections by transmitted
light, show a diffraction spectrum, due to the extreme delicacy
of the successive bands, whence they are termed rainbow agates.
On the disintegration of the matrix in which the agates are
embedded, they are set free, and, being by their siliceous
nature extremely resistant to the action of air and water,
remain as nodules in the soil and gravel, or become rolled as
pebbles in the streams. Such is the origin of the ``Scotch
pebbles,'' used as ornamental stones. They are agates derived
from the andesitic lavas of Old Red Sandstone age, chiefly
in the Ochils and the Sidlaws. In like manner, the South
American agates, so largely cut and polished at the present
time, are found mostly as boulders in the beds of rivers.
An enormous trade in agate-working is carried on in a small
district in Germany, around Oberstein on the Nahe, a tributary
of the Rhine at Bingen. Here the industry was located many
centuries ago, in consequence of the abundant occurrence
of agates in the amygdaloidal melaphyre of the district,
notably in the Galgenberg, or Steinkaulenberg, overlooking
the village of Idar, on the Idar Bach, about two miles from
Oberstein. The abundant water-power in the neighbourhood
had also a share in the determination of the industrial
site. At the present time, however, steam power and even
electricity are employed in the mills of the Oberstein
district. Although the agateindustry is still carried on
there, especially at Idar, the stones operated on are
not of indigenous origin, but are imported mostly from
Brazil (Rio Grande do Sul) and from Uruguay, where they
were discovered in 1827. Agate-working is also carried
on to a limited extent at Waldkirch in the Black Forest.
Most commercial agate is artificially stained, so that stones
naturally unattractive by their dull grey tints come to be
valuable for ornamental purposes. The art of staining the
stone is believed to be very ancient. Possibly referred to
by Pliny (bk. xxxvii. cap. 75), it was certainly practised
at an early date by the Italian cameo-workers, and from
Italy a knowledge of the art--long kept secret and practised
traditionally--passed in the early part of the 19th century to
the agate-workers in Germany, by whom it has since been greatly
developed. The colouring matter is absorbed by the porosity
of the stone, but different stones and even different layers
in the same stone exhibit great variation in absorptive
power. The Brazilian agates lend themselves readily to
coloration, while the German agates are much less receptive.
To produce a dark brown or black colour, the stone is kept perhaps
for two or three weeks in a saccharine solution, or in olive
oil, at a moderate temperature. After removal from this medium,
the agate is well washed and then digested for a short time in
sulphuric acid, which entering the pores chars or carbonizes
the absorbed sugar or oil. Certain layers of chalcedony are
practically impermeable, and these consequently remain uncoloured,
so that an alternation of dark and white bands is obtained,
thus giving rise to an onyx. If stained too dark, the colour
may be ``drawn,'' or lightened, by the action of nitric acid.
Agate is stained red, so as to form carnelian and sardonyx,
by means of ferric oxide. This may be derived from any iron
compound naturally present in the stone, especially from limonite
by dehydration on baking. Some stones are ``burnt'' by mere
exposure to the heat of the sun, whereby the brown colour passes to
red. Usually, however, an iron-salt, like ferrous sulphate,
is artificially introduced in solution and then decomposed
by heat, so as to form in the pores a rich red pigment.
A blue colour, supposed to render the agate rather like lapis
lazuli, is produced by using first an iron salt and then
a solution of ferrocyanide or ferricyanide of potassium;
a green colour, like that of chrysoprase, is obtained
by means of salts of nickel or of chromium; and a yellow
tint is developed by the action of hydrochloric acid.
Among the uses to which agate is applied may be mentioned the
formation of knife-edges of delicate balances, small mortars
and pestles for chemical work, burnishers and writing styles,
umbrella-handles, paper-knives, seals, brooches and other trivial
ornaments. Most of these are cut and polished in the Oberstein
district, at a very cheap rate, from South American stones.
Numerous localities in the United States and Canada yield
agates, as described by Dr G. F. Kunz. They are abundant in
the trap rocks of the Lake Superior region, some of the finest
coming from Michipicoten Island, Ontario. A locality on the
shore of the lake is called Agate Bay. Wood agate, or agatized
wood, is not infrequently found in Colorado, California and
elsewhere in the West, the most notable locality being the
famous ``silicified forest'' known as Chalcedony Park, in Apache
county, Arizona. Here there are vast numbers of water-rolled
logs of silicified wood, in rocks of Triassic age, but only
a small quantity of the wood is fine enough for ornamental
purposes. The cellular tissue of the vegetable matter is
filled, or even replaced, by various siliceous minerals like
chalcedony, jasper, crystalline quartz and semi-opal, the silica
having probably been introduced by thermal waters. Some of the
agate shows the microscopic structure of araucarian wood. The
agatized wood is sometimes known by the Indian name of shinarump.
In India agates occur abundantly in the amygdaloidal varieties of
the Deccan and Rajmahal traps, and as pebbles in the detritus
derived from these rocks. Some of the finest are found in the
agate-gravels near Ratanpur, in Rajpipla. The trade in agates
has been carried on from early times at Cambay, where the stones
are cut and polished. Agates are also worked at Jubbulpore.
In many parts of New South Wales, agates, resulting from the
disintegration of trap rocks, are common in the river-beds and old
drifts. They occur also in Queensland, as at Agate Creek, running
into the Gilbert river. South Africa likewise yields numerous
agates, especially in the gravels of the Orange and Vaal rivers.
It should be noted that in England agates are found not
only in old lavas, like the andesites of the Cheviots, but
also to a limited extent in the Dolomitic Conglomerate,
an old beachdeposit of Triassic age in the Mendips and
the neighbourhood of Bristol. They are also found as
weathered pebbles in the drift of Lichfield in Staffordshire.
For Scottish agates see M. F. Heddle, ``On the Structure of
Agates,'' Trans. Geolog. Soc. Glasgow, vol. xi. part ii.,
1900, p. 153; and Mineralogy of Scotland (1901), vol. i.
p. 58; J. G. Goodchild, Proc. Phys. Soc. Edinburgh, vol.
xiv., 1899, p. 191. For the agate-industry see G. Lange, Die
Halbedelsteine (Kreuznach, 1868). For American agates, G. F.
Kunz, Gems and Precious Stones of North America (1890), p.
