350 [B.N.F.C. 



and cavities of most volcanic lavas. We have, for example, the 

 zeolites in the basalts and dolerites, the agates and chalcedonies 

 in the andesites and trap-rocks, and also the opals and hyalites 

 in rocks of acid composition, such as rhyolites. 



In describing the formation of the deposits in the typical 

 Carnmoney vein, I have chosen them as a type of such for two 

 reasons ; firstly, because, as already explained, they exhibit 

 clearly the continuity between the igneous formation of the 

 mother-rock and the aqueous formation (hydro-thermal) of the 

 contained secondary minerals, and, secondly, because, as I now 

 briefly mention, they show, in beautiful sequence, the law of 

 order in the separation of secondary minerals in such lavas. 

 This law may be expressed in the following terms: — With the 

 fall of temperature in the residual waters of cooling volcanic 

 lavas there is a sequence of separation in the 'secondary' 

 minerals deposited from such waters in the veins and cavities 

 of the rock. These minerals fall into' three groups which are 

 deposited in the following order: — ist. Hydrous Earthy Sili- 

 cates, and Oxides (including the green earths, such as Celadon- 

 ite, Hullite, Chlorophaeite, Delessite, etc., and the Oxides of 

 Iron and Manganese) ; 2nd. Hydrous Crystalline Silicates and 

 Carbonates (including the Zeolite family, and the Carbonates of 

 Manganese, Iron, Magnesium, and Calcium) ; and 3rd, Crystal- 

 line and Hydrous Silica (including Quartz, Chalcedony, Hyalite, 

 and Opal). These three groups are frequently intergrown, and 

 often one, or two, of the groups are not represented at all. 



The typical Carnmoney vein exhibits all the three groups 

 distinctly, and is, on that account, a good type and example of 

 such formations. As further examples of the working of this 

 law, I shall now, in conclusion, mention the formation of several 

 Scottish deposits of chalcedony. The most common type of 

 chalcedony in the Scottish lavas is the agate with its wonderful 

 and varied formations. In this rase the outer layer is of green 

 earth, and is generally composed of celadonite (or celedonite), 

 chlorophaeite or saponite. Upon this we find the second layer, 

 which is occasionally of calcite, but commonly of the zeolites, 

 natrolite. and heulandite. The central portion of the cavity, 



