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 case 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, 
