QUARTZ (AGATE): OCCJURRENCE 513 



Agate occurs only rarely as veins filling the fissures of rocks. There are typical 

 examples of this mode of occurrence at Halsbach, near Freiberg, in Saxony, where the veins 

 consist for the most part of coral-agate, and at Schlottwitz, near Wesenstein, in the Muglitz 

 valley, in Saxony. The excellent riband-agate found at the latter locality has narrow, 

 brightly-coloured bands arranged parallel to the surfaces of the vein ; the latter contains, 

 besides agate, common chalcedony, jasper, quartz, and amethyst. At a certain spot the 

 material of one-half of the vein has been completely broken up by earth movements. The 

 angular fragments of agate thus produced were afterwards cemented together by amethyst, 

 the contrast in colour between these two minerals rendering the resulting stone, the well- 

 known hrecciated-agate, very effective for decorative purposes. The fragments of agate in 

 brecciated-agate sometimes suggest a representation of ruined buildings, this variety being 

 on that account referred to as ruin-agate. This brecciated-agate was discovered in 1750 

 and mined in some quantity. Like other Saxony agates, for example those found in 

 porphyry at Altendorf and Rochlitz, it was fashioned into all kinds of articles. A beautiful 

 rose-red ornamental sand was at one time prepared from the coral-agate of Halsbach, but 

 at the present time the mines are almost always filled with water and therefore inaccessible. 



Agate occurs much more commonly, however, in the vesicles of certain volcanic rocks, 

 such as porphyries and basalts, and specially in amygdaloidal melaphyres. These almond- 

 shaped or amygdaloidal cavities have already been mentioned under amethyst ; besides this 

 mineral most of them contain agate, and this, to distinguish it from the agate which occurs 

 in veins, is sometimes termed amygdaloidal agate. Almost the whole of the material 

 used for cutting is amygdaloidal agate. The cavities which are filled principally with agate 

 are called agate-am ygdales ; their external surface is usually very rough and pitted. 



The layers of agate of which these amygdales are built up usually follow more or less 

 closely the surface of the amygdale. When exactly parallel to the external surface we get 

 riband-agate, and when not exactly parallel we get one or other of the sub- varieties already 

 described. Many South American amygdales are remarkable in that the bands follow the 

 outline of the cavity for part of their course, and then, leaving the wall of the cavity, take a 

 short cut across it (Plate XIX, Fig. b). Usually in agates from all other localities, and 

 indeed in many South American amygdales also, the layers of chalcedony closely line the 

 walls of the cavity, as represented in Fig. a of Plate XIX. 



The agate of the amygdale is only rarely in immediate contact with the rock in which 

 it is embedded ; more frequently a thin layer of a green, earthy, chloritic, or micaceous 

 mineral intervenes, the same substance as the so-called green-earth, which occurs enclosed 

 in moss-agate and which imparts to plasma and heliotrope their green colour. The layers 

 of agate which succeed the layer of green-earth do not, as a rule, completely fill up the cavity 

 of the am} gdale, and such amygdales containing a central space are known as geodes. The 

 innermost layer of agate enclosing this central space may have a reniform or botryoidal 

 surface, as is common in all varieties of chalcedony, or it may hang in stalactites from the 

 roof of the cavity. Sometimes such a cavity is completely filled up by a later deposition of 

 agate, which encloses the stalactites of the former stage of growth and gives rise to the 

 so-called pipe-agate. 



Amygdales consisting entirely of agate are, however, very rare ; the innermost layer 

 surrounding the central space usually consists of crystallised quartz — often of amethyst — 

 the pyramidal terminations of the crystals projecting into the space. On the other hand, 

 the whole of the central cavity may be filled with crystalline quartz showing no crystal faces 

 (Plate XIX, Fig. h). In amygdales of this description thei-e may be only a small central 

 nucleus of amethyst, the bulk of the structure being of agate, or, again, the agate may form 



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