1905-1906.] 349 



second, at a low temperature, slow. By the first action such 

 minerals as the zeolites, chalcedony, and opal are deposited in 

 large quantities, whereas by the second action these substances 

 are formed in comparatively small amounts. By the second 

 process calcium, and magnesium carbonates are often deposited 

 extensively, because of their easy solubility as bi-carbonates in 

 cold, meteoric waters, but the deposits of the same minerals from 

 the hot residual waters of the cooling lava are often still more 

 extensive. In the case of the 'green earths,' we find that 

 minerals of this class, which are composed of the extremely 

 insoluble silicates of aluminium, iron, and magnesium with vary- 

 ing quantities of the alkaline silicates, are usually the first sub- 

 stances deposited from, the hot, residual waters of the lava. They 

 are found almost invariably coating the rock-sides of veins and 

 vesicular cavities of volcanic rocks. Such deposits are not ex- 

 tensive, because of the insolubility of these silicates. On the 

 other hand, we find vast deposits of "serpentinous" and 

 saponaceous green earths formed by the decomposition of 

 certain lavas. In some cases the whole rock has weathered to 

 a green earth. This substance consists chiefly of the above- 

 mentioned insoluble silicates, with small quantities of the 

 alkaline silicates held chemically-bound in the form of insoluble 

 double silicates, and is really the residue of the rock, the more 

 soluble substances having been removed by acid meteoric waters 

 in the course of weathering. This course is a slow one, and 

 most of the mineral matter thus removed is carried away in 

 spring and river waters. Re-deposition on a large scale is rare 

 in such rocks. When a volcanic rock has suffered from solfa- 

 taric action, or hydro-thermal metamorphism, such green earths 

 are also formed, not only as residues through the removal of the 

 more soluble elements in the rock, but, also where the latter are 

 re-deposited there is often a re-deposition of the green earths 

 in smaller quantities. 



9. The deposition of mineral matter from, the residual 

 waters of volcanic magmas, which we have regarded as the 

 source of the Carnmoney chalcedony, accounts for the formation 

 of the main masses of "secondary' siliceous minerals in the veins 



