AGATE 33 



originally formed by the disengagement of gas or steam at a time when the melaphyre 

 was in a molton or partially molten state. The formation of such vesicles in a plastic 

 mass may be well illustrated by the spongy texture of a loaf of bread. Originally the 

 form of the bubbles in the viscid rock would bo more or less globular, but by move- 

 ment of the pasty mass the hollows might become elongated, or, as often happens, 

 pointed at one extremity ; if the mass were slowly moving upwards the point would 

 be directed downwards. In many cases the cavities have been much extended and 

 laterally compressed ; and hence the agates now occupying such hollows are elongated, 

 flattened and pointed, thus resembling almonds, and the rock containing them is con- 

 sequently termed amygdaloidal melaphyre. Evidence of the movement of the viscous 

 rock-mass is further afforded by the parallelism often observable in the amygdaloidal 

 agates as they lie in the rock the longer axes of these agates being all arranged in 

 one direction. The smaller agates are often perfectly amygdaloidal, but the larger 

 specimens are usually distorted. It is likely that the cavities of the large amygdaloids 

 may have been produced by the coalescence of several smaller vesicles. 



On the cooling of the igneous mass, water charged with carbonic acid would 

 percolate through the rock, and effect the decomposition of some of the mineral con- 

 stituents the chemical changes being perhaps aided by the heat still lingering in the 

 rock. The products of this decomposition might be carried into the vesicular cavities, 

 and thrown down as a lining on their inner walls. Among the first formed of such 

 products are the minerals called delessite, or ferruginous chlorite, and the somewhat 

 similar substance termed 'green earth ; ' many vesicular cavities exhibit nothing more 

 than a layer of such green minerals, derived probably from the decomposition of the 

 augite in the rock, whilst most agates especially the smaller ones when freed from 

 their matrix present a green external coating of a like nature. In other cases a 

 hydrous peroxide of iron appears to have been formed, perhaps by further alteration 

 of the green earth ; and hence many large agates present a rusty exterior or exhibit a 

 pitted surface due to the former presence of a coating of this mineral. The smaller 

 cavities in amygdaloidal rocks are commonly filled with carbonate of lime, and the 

 solid nodules removed from such rocks consequently appear as small green-coated 

 masses of calcite. But in most cases silica has been separated from the silicates 

 decomposed in the rock, and has been thrown down in a gelatinous state on the walls 

 of the cavity as a coating of chalcedony. This coating may be nothing more than a 

 thin rind, thus forming a hollow agate or geode. It generally happens that crystalline 

 silica is deposited on the inner surface of the chalcedonic layer, which thus bears a 

 crop of crystals of quartz the free pointed ends of these crystals being all directed 

 towards the centre of the geode. Fine amethysts are often found seated in such 

 situations. In other cases the cavity may contain stalactitic deposits of chalcedony, 

 or crystals of calcite, or of various hydrated silicates called zeolites, and rarely of 

 certain metallic minerals as iron-glance, copper-pyrites, native copper, &c, 



By continued deposition of chalcedony, or of alternate layers of chalcedony and 

 crystalline quartz, the cavity may become completely filled, and a solid agate thus 

 formed. 



Many agates exhibit, on section, tubular orifices, which are commonly supposed to 

 have served as inlets through which the agate-forming materials have been introduced 

 by infiltration. In some specimens these inlets have become choked up at an early 

 period in the history of the stone, and the introduction of fresh matter was thus 

 prevented ; in other cases the tubes have remained open until the cavity was com- 

 pletely filled, and connexion has thus been maintained between the very heart of 

 the agate and the exterior. This ' infiltration theory ' has been staunchly supported 

 by Von Buch and Noggerath. On the other hand, Haidinger supposed that the silica, 

 instead of being introduced through special apertures, exuded through the general 

 walls of the cavity. After a lining had been laid down uniformly over the interior, 

 more silica in solution might pass from the exterior through this layer by a kind of 

 osmosis, and thus gain access to the interior. But in describing the modern method 

 of colouring agates, it will be shown that certain layers of chalcedony are quite 

 impermeable to fluids, at least under ordinary conditions of temperature and pressure. 

 As soon, then, as a dense impervious layer was deposited, it would seem at first sight 

 that all action must cease ; but it has been suggested that siliceous solutions might 

 still find their way to the interior through the cracks with which agates are almost 

 invariably rent. 



A different theory of agate formation has been advanced by Eeusch. He supposes 

 that warm siliceous solutions were from time to time introduced into the cavities by 

 the action of intermittent thermal springs, the cavities being thus alternately filled 

 and emptied. The intermission of the action may account for the definite succession 

 of the deposits, and the sharp lines of separation between the several strata. This 

 theory has been ingeniously extended by Lange, who seeks to explain the formation of 



VOL. L D 



