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Sec. IV, Ch. 1: Semi-Precious Stone, Agate

Sec. IV, Ch. 1: Semi-Precious Stone, Agate Page of 366 Sec. IV, Ch. 1: Semi-Precious Stone, Agate Text size:minus plus Restore normal size   Mail page  Print this page
236                                   The Agate.
Various theories have been propounded from time to time, for the purpose of explaining the origin of the Agate nodules in the cavities of the rocks wherein they occur. The cavities themselves have unquestionably resulted from the imprisonment of gas bubbles, whilst the rock was in a molten condition. The agate-bearing rock is, in most cases, an ancient lava. The nodules of Agate are con­sidered to result from the crystallization, or non-crystalline deposition, of silica, from a solution with which the cavity of the nodule or geode became filled. The silica—now in one condition, such as Jasper, now in another, such as Chalcedony, and then again in the crystallized form of Quartz—was deposited over the irregular inner surface, giving rise to those concentric markings which are seen on the sections of most Agates. This deposition of silica would continue until the geode became filled so as to form a solid Agate, or the inlets of infiltration became stopped up, or the supply of siliceous solution failed. In other cases the silica would be deposited on the walls of the cavity in concentric layers, while, after a time, owing to some change in the natural conditions, the silica might be deposited in layers on the floor of the cavity, in obedience to gravita­tion, and the various coloured bands would then run parallel to each other in horizontal layers.
According to certain fancied similitudes, which the Agate stone displays to things in common use, it receives distinguishing names. Thus Riband Agate exhibits strata or layers of different colours which play one into the other. If the stripes of varied hues are arranged round the centre, it receives the name of Circular Agate ; and if in this centre there are other coloured points, it is called Eye Agate. When the variously coloured bands are disposed in an angular pattern, suggestive of the plan of a polygonal
Sec. IV, Ch. 1: Semi-Precious Stone, Agate Page of 366 Sec. IV, Ch. 1: Semi-Precious Stone, Agate
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