On the Genesis of Some Scottish Minerals. 197 



tion that the solutions have carried in chalcedonic materials 

 alone. In actual fact this is rarely the case. The chalcedonic 

 matter has often been deposited as a mixture, sometimes with 

 zeolitic matter, in which case the material becomes more or 

 less opaque on consolidation, and usually somewhat chalky 

 in aspect. This, in the case of chalcedonic matter, may be 

 called chalcedonic Cachalong. But the solutions are by no 

 means always chalcedonic. Taking Scottish agates as a 

 whole, one may say that nearly a fourth of the silica carried 

 in consists of colloid or quite amorphous silica, combined 

 with a variable quantity of water, which may range to twelve 

 per cent, or even more. Such a compound is known as 

 Opal. It has already been mentioned as forming the basis 

 of Chalcedony — which is Opal minus most, or all, of its 

 water, and plus a variable percentage of crystalline or 

 crystalloid silica. There seems reason, therefore, for regard- 

 ing Chalcedony rather as a mixture than as a compound 

 having a definite composition. In a few cases, and especially 

 towards the end of the period of agate formation, the opaline 

 constituent diminishes to nothing. In that case Quartz or 

 Amethyst results. 



This variability in the percentage of opaline silica carried 

 in is of considerable importance in the history of an agate. 

 Solutions depositing pure Opal conform wholly to the laws 

 of Gravitation, and are in nowise affected by Surface Tension. 

 That is to say, a solution depositing Opal- Silica does not coat 

 the sides or the roof of an agate cavity; but the layers 

 subside entirely to the bottom, and arrange themselves in 

 absolutely horizontal positions, even at their edges, where 

 they abut against the sides of the cavity. The deposition of 

 Chalcedonic Silica, it will be remembered, is controlled more 

 by Surface Tension, and may therefore be deposited in even 

 larger quantities on the roof or the side of the cavity than 

 on its floor. Mixtures, in variable proportion, of opaHne 

 silica with chalcedonic silica commonly occur. Where the 

 opaline silica predominates, Gravitation comes most into 

 play. Where the chalcedonic silica forms the chief ingredient 

 in the mixture. Surface Energy prevails. Now, it is quite 

 clear from the study of large numbers of agates, such as those 



