A'GATE, an ornamental stone used in jewellery, and for some purposes in the arts : it is sometimes called Scotch Pebble. The name is derived from the Greek axdrer, a stone described by Theophraatus, and which, he says, came from the river Achates, in Sicily, now the Drillo, in the Val di Note. It is one of the numerous modifications of form under which silica presents itself, almost in a state of purity, constituting in the agate 93 per cent. of the mineral. The silicious particles are not so arranged as to produce the tran sparency of rock crystal, but a translucent, sometimes almost opaque substance, with a resinous or waxy fracture; and a variety of shades of colour are produced by a minute quantity of iron. The same stone sometimes contains parts of different degrees of translucency, and of various shades of colour; and the endless combinations of these produce the beautiful and singular internal forms, for which, together with the high polish they are capable of receiving, agates are prized as ornamental stones. Although occasionally found in other rocks, they are most usually met with in that variety of the trap rocks called Amygdaloid or Mandelstein, forming detached rounded nodules, not cemented to the base or mass of the rock, but easily separable from it, and having generally a thin layer of green earth interposed, and a rough irregular exterior, as if moulded on the asperities of the sides of a pre-existing cavity. The silicious particles have often, but far from constantly, arranged themselves in thin layers parallel to the external surface of the nodule ; sometimes the nodule is not solid, but a hollow space is left in it, studded with crystals of quartz ; and not unfrequently crystals of carbonate of lime and other minerals, totally distinct in composition from that of the agate, are superimposed on the quartz crystals.
The theory of the formation of agates is a problem of great difficulty, and we must be much further advanced than we are, in our knowledge of the chemical processes of nature in the mineral kingdom, before we can expect to throw any light on this very obscure subject. The great supply of agates is from a class of rocks to which all geologists now assign an igneous origin, analogous to that of lava in existing volcanoes. The theory divides itself into two parts ; first, the forma tion of the cavities in which the agates are found ; and, secondly, the filling of these cavities. With regard to the first, we have many
analogies from modern lavas, and from processes of art, to guide us to a pretty satisfactory concluSion. Gases are evolved in great quantities by volcanoes, and if produced at the same instant with a flow of lava, they would rise in bubbles in the melted mass ; but in proportion as that became more viscid they would rise with greater difficulty to the surface, and when it consolidated would form cavities, the shape of which would be determined by the nature of the pressure of the surrounding viscid lava. To account for the filling up of the cavities three theories have been proposed : one supposes the officious matter to have been introduced in aqueous solution from without, and to have been gradually deposited in the cavities ; another, that, in obedience to some peculiar laws of attraction, it has separated from the rest of the rock, and insinuated itself into the hollows left by the gases; and a third, that these hollows were filled by the sublimation of the silica and other materials from the rest of the mass by the action of heat. Each hypothesis is supported by particular eases, which it satisfactorily explains, but there are probably as many against as in favour of each ; all of them imply conditions of chemical action different from anything of which we have had experience. We fre quently find, it is true, masses of silicious petrified wood in which hollows of the tree have been filled with agate, not to be distinguished from many nodules found in the trap rocks ; and that the matter of the agate must have been introduced into the wood by aqueous infiltrations there can be no doubt : but, in this case, the whole substance of the sustaining mass, the wood, is by silicious matter; and the difficulty of the theory of infiltration, in the ease of the trap rocks, consists in the absence of any trace in the rock of the channel by which the solution of silicioua matter could have arrived at the cavity. The following section of an agate is a good example of the filling up of a cavity by infiltration, for it is evident that the sili ceous matter, in whatever way it may have arrived, was introduced at the point a, and that there was a gradual deposition of it.