344 [b.n.p.c. 



during the last twenty years by J. G. Goodchild, and to-day we 

 find a similar opinion in the fine text-book by Professor Miers. 



Other writers, still pointing to rock-decomposition as the 

 source of this 'secondary silica,' regard it as more or less a 

 product of 'solfataric' action formed soon after the solidifica- 

 tion of the lava. But even the standard text-books of geology 

 are strangely indefinite in their remarks on this subject. The 

 Carnmoney chalcedony is a very near relation to* the agates 

 found in the Scottish andesitic lavas, and bearing this in mind, 

 we may express modern opinion as divided into two classes of 

 theories concerning the origin of chalcedony in lavas. These 

 are : — < 



(i). The Hydro-thermal or Solfataric Theory. This 

 theory was applied to the Carnmoney chalcedony by Mr. 

 Gault, who supposed that, long after the volcano has spent its 

 forces, hot alkaline water was poured into- the veins and per- 

 colated through the mass of the rock. These hot springs dis- 

 solved silica and other substances out of the rock-mass, and 

 re-deposited them in the cavities and veins. He thus accounted 

 for the origin, not only of the chalcedony, but also of the hul- 

 lite and other minerals. Gault seems to have considered that 

 a common origin must be sought to account for the formation 

 of all the minerals found in the veins. 



(2). The Epigenetic Theory. The theorists of this class 

 hold that the chalcedony is a decomposition-product formed 

 during the weathering of the mother rock. Rain-water falls 

 upon the ground, and passing through the soil, which contains 

 decaying vegetable matter, becomes charged with carbonic acid 

 gas, and humic acid, which gives rise to carbonic acid. The 

 dilute solutions of carbonic acid slowly percolate through the 

 rock-mass, attacking the silicates, and become charged with 

 carbonates of the alkalies and alkaline earths, and free silica. 

 Labradorite, for example, loses sodium and calcium., which are 

 removed as carbonates, and silica as silicic acid, leaving silicate 

 of alumina behind in the form of clay. The calcium carbonate 

 and silica are deposited in such cavities or veins as occur in the 



