34 AGATE 



l>oth concentric and horizontal layers in the same stone, an association often observed 

 iu the South American agates, and always difficult of explanation. Lango maintains 

 that when the gelatinous silica, deposited in a warm state, had choked up the entrance, 

 the tension of the steam confined in this closed cavity would causo the siliceous jelly 

 to bo pressed equally in all directions against the inner surface of the cavity, thus 

 forming a continuous lining, until the steam finally burst the coating at its weakest 

 point, and so effected ite egress : the so-called inlets of infiltration may therefore bo really 

 canals of eruption. If the cavity were largo, the elasticity of the vapour might bo 

 insufficient to press the gelatinous matter against tho sides of the hollow, and the 

 contents would then bo precipitated, in obedience to the force of gravity, in horizontal 

 strata on the floor of the cavity : hence the co-existence of concentric and horizontal 

 layers in a single agate. 



According to the varying conditions under which the successive strata of an agate 

 have been formed, different varieties of the mineral are produced, and many of these 

 are sufficiently well characterized to receive special names. Thus, when tho cavity in 

 which tho stone has been formed presents angular contours, the layers naturally adapt 

 themselves to these angles, and the cut stone thus exhibits a zigzag pattern, whence 

 it is termed fortification agate. If the deposits form concentric rings the stone is 

 called eye agate. Such trivial names as ribbon agate, landscape agate, clouded agate, 

 and the like, sufficiently explain themselves. A beautiful brecciated agate, in which 

 angular fragments of a banded variety are cemented together, chiefly by amethyst, is 

 well known from Schlottwitz in Saxony. 



The colours of agate are arranged in parallel or concentric bands, or assume 

 the form of clouds or spots, or arborescent and moss-like stains. These colours are 

 due to the presence of metallic oxides. When black and white strata alternate, the 

 stone is called an onyx ; when the layers are brown, or red and white, it becomes a 

 sardonyx. If the white stratum of an onyx be so thin as to appear bluish white, the 

 stone is termed by jewellers a nicolo. The mocha stone is a variety containing 

 dendritic markings due to the presence of oxide of manganese, or of iron, whilst the 

 moss agate is a chalcedony, containing green moss-like markings, referable to included 

 inorganic colouring-matter disposed in these patterns. 



The chief localities which yield agates to any extent are the melaphyro rocks in 

 the Galgenberg and elsewhere in the neighbourhood of Oberstein in Rhenish 

 Bavaria, and the beds of the Kio Pardo, tho Taquarie, and other rivers in Uruguay, 

 which yield the nodules commonly called ' Brazilian agates.' Fine Oriental agates 

 are imported from India, chiefly from Cambay, where they are largely cut and 

 polished. The well-known ' Scotch pebbles ' are true agates found in various localities 

 in Scotland, especially in the amygdaloidal rocks of Kinnoul Hill in Perthshire ; near 

 Montrose in Forfarshire ; at Dunbar ; at Dunglass in Haddingtonshire, and elsewhere. 

 The pebbles found on the south coast of England are not true agates, but are simply 

 flints derived from the upper chalk, and often exhibit patterns due to tho presence of 

 choanites, ventriculites, and other organic remains. At the same time large numbers 

 of agates are also sold, at low prices, at most English watering-places, but these, so 

 far from being local ' pebbles,' are generally South American agates, cut and polished 

 in Germany, whence they are largely imported. Fine pebbles of agate and other 

 siliceous stones are found in the Vaal River in South Africa, and also in the Nile. 

 Mr. Daintree has recently described a local eruption of melaphyre, parts of which 

 contain fine agates, at Agate Creek, a tributary of the Gilbert Eiver in Queensland. 

 Some beautiful polished examples of Queensland agates were exhibited in the Inter- 

 national Exhibition of 1872. 



Agates are used in the arts for a variety of purposes, such as knife-edges of delicate 

 balances, small mortars for chemical purposes, burnishers for gold and silver, stylos 

 for writing, seal-handles, brooches, bracelets, beads, and an endless variety of small 

 ornamental objects. 



These hard stones are cut and polished almost exclusively in a small district at tha 

 foot of the Southern Hundsruck in Western Germany. The works are chiefly situated 

 along the Valley of the Idar, a small stream which flows into tho Nahe. At a distance 

 of about 40 miles from Bingen, where the Nahe empties itself into tho Rhine, tho 

 small town of Oberstein is situated. Oberstein and Idar about two miles distant 

 are the chief centres of the agate trade. Although it is true that there are many 

 other localities where agate-working is carried on to a limited extent, it is only in 

 this district at least in Western countries that the trade is systematically pursued, 

 and forms the staple industry. From the low value of labour, the agates aro there 

 cut and polished at incredibly low prices, and vast quantities of the cut stones aro 

 sent thence to all the continental fairs and watering-places. A large trade is also 

 carried on with London, Birmingham, Paris, New York, and other distant localities ; 

 considerable quantities, too, are exported to the interior of Africa. 



