STRUCTURES COMMON TO ALL ROCKS. 235 



the larger portion is usually vein-stuff ; and through this 

 is disseminated the metallic ore in granules, strings, or 

 larger masses (Fig. 140, c), or sometimes in a central sheet 

 (Figs. 139, 141), as if de- 

 posited last of all. The 

 principal kinds of vein-stuffs 

 are silica, carbonates of lime, 

 iron and baryta, sulphate of 

 baryta, and fluoride of cal- 

 cium (fluor-spar). Often, 

 however, many kinds of 

 minerals are aggregated into 

 a veritable vein-rock. The most common of all is silica, 

 in the form of quartz. Next comes lime-carbonate. The 

 metals sometimes occur free (M), as, for example, always 

 gold and platinum, often silver, and sometimes copper and 

 mercury. But more commonly they occur as metallic 

 sulphides (MS), carbonates (MC0 3 ), and oxides (MO). 

 By far the most common form is sulphides. These facts 

 are given in the schedule. The most abundant kinds are 

 marked with a -f . 



Structure. Veins have nearly always a more or less 

 banded structure, as if the materials were deposited in 

 successive layers, on the two sides alike. Sometimes the 



bah abac 



FIG. 139. a, central sheet of ore ; 

 bb, agate ; <?, wall-rock. 



FIG. 140. aa, agate ; 6, quartz ; c, 

 copper-bearing lode ; d, wall-rock. 



successive layers are of the same material, but of different 

 colors (Figs. 139, ~b~b, 140, aa]\ sometimes of different ma- 



