19051906.] 547 



occasionally minute tufts of zeolites may be found in similar 

 positions. There is no doubt that cold, surface-waters, charged 

 with carbonic acid, can remove calcium carbonate from the 

 rock, and dissolve existing crystals of calcite, carrying away, in 

 the form of bi-carbonate, considerable amounts of that mineral, 

 and re-depositing it in other places. The formation of zeolites 

 from felspar and from existing zeolites takes place in a similar 

 manner, but to a much less extent ; and the formation of 

 chalcedony in this fashion appears to be a very rare occur- 

 rence. There is also no doubt that these reactions, resulting in 

 the growth of calcite, hydrated silicates, and various forms of 

 silica, would go* on at a much faster rate, and to a much greater 

 extent, if the cold meteoric waters were replaced by hot alkaline 

 waters of telluric origin. These actions have their place in 

 nature; but when we consider the molten magma from which 

 certain lavas have been formed, containing water under immense 

 pressure, and at a temperature far exceeding that of any 

 solfataric water, we come to the conclusion that the residual 

 waters of such magmas contained large quantities of silica, 

 silicates, and carbonates in solution. As the temperature fell, 

 the dissolved mineral matter would be secreted and deposited 

 in the veins and cavities, in a definite sequence, according to 

 the nature of the dissolved elements. It is natural that such 

 secondary minerals should bear some chemical relation to the 

 primary rock-forming minerals, because both are formed from 

 the same magma. 



The rock-forming minerals of the Carnmoney basaltic 

 andesite are augite, labradorite felspar and magnetite. Augite is 

 composed mainly of calcium, magnesium, aluminium, and iron 

 silicates, the silica amounting to 47-48 per cent. In the hullite 

 we find hydrous silicates of the same elements, iron being the 

 preponderating element. When the water of hydration is 

 omitted from the analysis the silica amounts to almost 46 per 

 cent. The primary augite and magnetite have thus a second- 

 ary counterpart in the hullite. Labradorite is composed of 

 aluminium, calcium, and sodium silicates ; the silica amounting 

 to 5 2 -53 per cent. In natrolite (the most common zeolite in 



