136 THE ORIGIN AND HISTORY OF 



Studded over with thousands of minute quartz crystals, and then 

 on these several dog-tooth calc-spar crystals have been deposited. 



The theory that the various layers have been deposited by 

 silica in solution entering through an aperture has been objected 

 to, and another has been proposed and strongly advocated by Dr. 

 Haidenger in Germany and Prof. Heddle and others in England. 

 Their theory is that the silica enters the cavity in solution by a 

 general exudation through its walls in a colloid or gelatinous con- 

 dition, the first supply leaving a thin coating all round on the sides 

 and roof as well as on the floor ; while the centre of the cavity is 

 filled with water more or less pure. A fresh supply of a strong 

 solution, and by a process of dialysis, passes through the first or any 

 number of thin skins or layers, and as the layers increase forces 

 out the water through the channel which has been supposed to be 

 the point of entrance. An objection that some of the layers of 

 agate are impermeable and would not admit of the passage of the 

 silica through them, has been met by the fact that the microscope 

 shows that the concentric circles are crossed by radiating crystals 

 of tridamyte, through which the silica, and also various colouring 

 matters pass towards the centre of the cavity. But perhaps the 

 layers which now appear impermeable were not so originally. 



Some agates are entirely filled up by concentric layers ; others 

 by concentric and horizontal layers ; and others, again, have, in 

 addition to these, a portion filled with compact chalcedony or with 

 quartz crystals. The concentric layers were evidently first pro- 

 duced by a slow process ; the horizontal layers may have been the 

 result of a more rapid accession of silica, or, as some scientists 

 think, by chemical precipitation on the floor of the cavity by the 

 introduction of an acid, as CO2, into the cavity. The remaining 

 portion, by an influx of silica, being filled in the one case with 

 chalcedony and in the other with quartz crystals formed by silica 

 held in solution, crystallising in the unfilled portion of the cavity. 



The deposition of the successive layers of silica has not always 

 proceeded from the outside inwards ; frequently, it appears to have 

 proceeded from fixed centres by a radiating or stalactitic process. 

 The action of acids on gelatinous or colloid silica appears to have 

 the effect of producing slender tubes or [lipettes of consolidated 

 crystalline matter, and gradually depositing layer upon layer 



