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agate-holding traps are all rotten ; the colloidal silica, with a 

 certain quantity of tridamyte, is taken up by this water, and 

 transfuses into the cavity ; the silica is there solidified — probably 

 the layer of delessite is the coagulation. We have now a cavity 

 slightly lined with chalcedonic matter, containing, within, 

 water more or less pure, while without (that is outside of the 

 now double skin, delessite and first layer) we have a strong 

 solution of colloidal silica constantly supplied. Endosmose and 

 exosmose are set up with all their resistless force. The strong 

 solution finds its way through the two or any number of 

 increasing skins: the weak water is forced out through the 

 " point of infiltration," and so in its passage out thins all the 

 successively deposited layers at that place. By this continuous 

 flow of colloidal silica (held in solution by liquid) through the 

 already coagulated or deposited layers, continuous coagulation 

 of the silica in the yet hollow agate, and continuous extrusion 

 of the residual water, we have the ultimate filling up of the 

 cavity, and a solid agate formed. 



The adhesion of agates to the containing rock is slight in 

 most cases, from the so-called " skin" being magnesian and 

 soapy. 



The " point of infiltration," instead of being at once filled up, 

 as would result from the inflow of coagulable silica, is in reality 

 the last point filled up, being truly the point of escape : indeed 

 it frequently is not altogether filled up, remaining an open 

 tube. 



The microscope shows on a cross section the concentric layers 

 of coagulated silica, soluble in alkalies ; the crystals or fibres of 

 tridamyte cross these layers at right angles, radiating like a 

 rheolite from the skin, and it is always along the sides of these 

 crystals that intruding and staining liquids find a way ; pro- 

 bably, therefore, along their sides also did the ingress of chal- 

 cedonic fluid find entrance. 



I remain very truly yours, 



M. FORSTER HEDDLE. 



