132 THE ORIGIN AND HISTORY OF 



acicular or needle-shaped crystals, radiating from a centre, and 

 giving to polished specimens a beautiful mammillated or mottled 

 appearance. Chalcedony has a waxy lustre, is semi-transparent to 

 translucent, and of various colours. The tendency to a reniform 

 structure is supposed to play an imjiortant part in forming the 

 circular and curved lines in agates. These are also affected by 

 the stalactitic process. 



Quartz in agates is pure silica, crystalline, as in vein quartz, 

 and crystaUised, as in rock crystal, generally occupying the centre 

 of the cavity, but occasionally the outer rim and intermediate 

 zones. The strong tendency of quartz to crystallise in straight 

 lines and form true hexagonal prisms interferes with the circular 

 force exerted by chalcedony, and promotes the formation of irre- 

 gular figures in agates. 



Jasper is a compact, amorphous, or non-crystalline, opaque 

 variety of quartz, having in some kinds a dull earthy, and in 

 others a conchoidal, or partially conchoidal, fracture. The opacity 

 is the result of alteration and the addition of oxides of iron and 

 other impurities. Some heliotropes or jasper bloodstones, being 

 chalcedonic varieties, are translucent at the edges. Many jaspers 

 are altered or metamorphosed sandstones, and porcelain jasper is 

 merely baked clay or shale, produced either by volcanic or plu- 

 tonic action, or by the burning of beds of coal in close proximity. 

 Ribbon jasper consists of parallel layers of various shades of red, 

 green, white, and yellow colours. 



Egyptian Jasper occurs in nodules in the mud of the Nile and 

 in the sandy deposits in the district around Cairo. Central bands 

 divide the nodules into zones of various shades of brown colour, 

 the central zone being generally light brown, separated from other 

 lightish-brown zones by darker bands. These zones are supposed 

 to result from decomposition and subsequent colouring, in the 

 same way as zones in clay-ironstone nodules are produced. Prof. 

 Ruskin says jasper is eminently retractile, like the clay in septaria, 

 and in agates often breaks into warped fragments, dragging the 

 rest of the stone into distortions. In general, the embedded 

 fragments in any brecciated agate will be mainly of jasper, the 

 cement chalcedonic, or quartzose. 



Opal — which is an occasional constituent of agate — is an 



