1905-1906.] 
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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. SJlch 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 Cammoney chalcedony, accounts for the formation 
of the main masses of ‘secondary’ siliceous minerals in the veins 
