On the Genesis of Some Scottish Minerals. 195 



standing nearly all the structural features of agates ; while a 

 clear conception of the operation of Osmosis gives us an 

 equally trustworthy clue to the history of the enigmas pre- 

 sented by the remainder. 



With the completion of the Clear Chalcedony Layer there 

 often arose yet another change. Probably the earlier-formed 

 layers had by this time consolidated, more or less. The 

 change referred to is the formation of minute tufts, probably 

 representing a second crop of zeolites, which arose, sporadic- 

 ally, here and there upon the " skin." Their precise nature 

 does not appear to have been yet made out, not even by 

 Dr Heddle, who had thousands of sections of agates cut for 

 microscopic examination, and had microscopes specially made 

 for this work. Minute as they certainly were, in every case 

 their effect upon the succeeding structures must have been 

 very important. We may realise the principle upon which 

 these effects depend, by remembering that Surface Energy 

 acts proportionately to the surface - area concerned. If, 

 within a given space, the surface-area can be increased rela- 

 tively to the volume of space within which it stands, the 

 effects of Surface Tension will be increased in proportion. 

 A loosely-rolled ball of worsted will present a total surface- 

 area many times in excess of that of a wooden ball of 

 the same diameter ; and the area presented by the leaves 

 and stems of a bush may be more than a hundredfold that of 

 the surface-area of a box in which the bush could be packed 

 without compression. If, therefore, even a small roughness 

 should arise upon an area elsewhere smooth, any solutions 

 tending to deposit upon the whole area would deposit in 

 larger quantities where the rough part of the surface pre- 

 sented itself than where it was smooth. In accordance with 

 this principle, it will be readily understood that any chalce- 

 donic solutions carried into a cavity where even minute tufts 

 had arisen would deposit in larger quantities over them than 

 elsewhere. And as Surface Energy constantly tends to a 

 minimum, the form of each succeeding deposit tended to 

 become that of a hemisphere, because that form of surface 

 offers the smallest area of any solid of the same dimensions. 

 Therefore, each succeeding coat of Chalcedony tended to 



