On the Genesis of Some Scottish Minerals. 193 



In some cases little zeolitic matter, or even none, was de- 

 posited, and the layer was usually very thin ; but the deposit 

 resembled that of the Celadonite in taking place uniformly 

 over every part of the wall of the cavity. Occasionally the 

 deposition of the zeolitic matter went on until the greater 

 part or even the whole of the cavity was filled. This has 

 often happened in one cavity close to another in which the 

 deposit from first to last has been that of the Celadonite. In 

 other cases it may be close to another cavity which contains 

 a perfectly-formed agate. It is by no means clear why 

 adjacent cavities, penetrated, it is to be supposed, by solutions 

 which are alike, should, in one case, contain chalcedony, in 

 another — distant, perhaps, only a fraction of an inch — should 

 contain a serpentinous mineral, while a third may contain an 

 agate, a fourth Calcite, and a fifth remain unfilled. Yet such 

 is the fact. 



Leaving this to be explained by the results of future in- 

 vestigations, we may revert to the third layer in the order of 

 deposition, the Clear Chalcedony Layer, already noticed inci- 

 dentally. Whether the Celadonite layer, or the Zeolite 

 layer, be absent or not, this Clear Chalcedony Layer appears 

 to be present in most, if not in all, agates. It uniformly coats 

 every part of the cavity, and follows all the ins and outs of 

 its surface, to whatever cause these inequalities may be due. 

 This curious fact of a material deposited from solution ad- 

 hering to both the roof and the sides of a cavity, as it were 

 in defiance of gravitation, is regarded as due to the powerful 

 surface tension which exists, on the one hand, between those 

 solutions of silica which are destined to become Chalcedony, 

 and each of the three substances upon which it may be 

 deposited, on the other. It is well known that there is a 

 difference of degree in the surface-tension between any single 

 fluid and a solid of any other kind; or, conversely, that 

 between any solid and any one of all other fluids; the 

 surface tension being often greatest between any given solid 

 and a fluid tending to deposit crystalline matter. Chalcedony 

 is usually regarded as a mixture of colloid silica — virtually 

 Opal — with crystalloid or crystalline silica, which may be 

 Quartz, or Tridymite, or, it may be, a third form of crystallised 



