236 



STRUCTUKE COMMON TO ALL ROCKS. 



sulphate of baryta (heavy spar), and fluoride of calcium (fluor-spar). 

 By far the most common of these is quartz, and next is calc-spar. 

 Often, however, the vein-stuff is an aggregate of minerals forming a 

 true rock. Nearly the whole of a vein consists usually of vein-stuff. 

 The ore exists in comparatively small quantities, sometimes forming a 

 central rib or sheet, as if deposited last (Fig. 208. a b) ; sometimes in 

 irregular isolated masses called bunches or pockets, or in small strings, 

 or grains, irregularly scattered through the vein-stuff and extending 

 often a little way into the wall-rock. 



The chemical forms in which metals occur are very various; some- 

 times they occur as pure metal (as always in the case of gold and plat- 

 inum, and sometimes in the case of silver and copper), but more com- 

 monly in the form of metallic sulphides, metallic oxides, and metallic 

 carbonates. Of these the metallic sulphides are by far the most com- 

 mon. It is worthy of remark that all these forms are comparatively 

 very insoluble. The same is true of the vein-stuffs. 



Ribboned Structure. — The ribboned or banded structure, already 

 spoken of under Veins of Infiltration, is very commonly found in great 

 fissure-veins. This structure is as characteristic of veins as the. colum- 

 nar structure is of dikes. The layers on the two sides usually corre- 

 spond to each other (Fig. 208) ; sometimes the successive layers are of 

 different color, giving rise to a beautiful, striped appearance. Some- 

 times the successive layers on both sides are of different materials, as 

 in Fig. 209, in which the central rib, d, is galena, and a a, b b, c c, are 





I 





wa.hceLc'ba. 



Fig. 209. 



Fig. 210. 



successive layers of quartz, fluor, and baryta. Sometimes, in cases of 

 quartz-filling, the layers are agate, except the center, which is filled up 

 with a comb of interlocking crystals, as in Fig. 210. The same occurs 

 often in amygdules, the last filling being crystalline. Sometimes there 

 is evidence of successive openings and fillings, as in Fig. 211, where a 

 represents quartz-crystals, interlocking in the center and based on agate 

 layers, b b, while c represents quartz with disseminated copper pyrites. 

 In this case it seems probable that 1 and 2 were the walls when the 

 agate and quartz-filling took place, and that afterward the fissure was 

 reopened along 2, so that the walls became 2 and 3, and the new fissure 



