STRUCTURE OF VEINS. 401 



appear, under certain definite relations to the enclosing rocks as well 

 as to the included metals, and with a less distinct dependence on the 

 local situation or mining district. These earthy substances are usually 

 called the gangue, vein- stuff, or matrix of the ore. Generally they are 

 crystallised, as quartz, fluor spar, calcareous spar, phosphate of lime, 

 the sulphate and carbonate of baryta, strontian, &c. ; sometimes they 

 appear massive, as quartz and several other minerals, when the vein 

 has no cavities in it ; and sometimes the vein-stuff is entirely soft 

 argillaceous matter, of different aspect in different mining districts. 



Rider. In some veins masses of the neighbouring rocks are en- 

 closed and penetrated to a great extent by little strings of the ore and 

 spar, so as occasionally to be worth the trouble of working. The vein 

 is said in this case to bear a rider. It, in fact, sometimes becomes 

 under these circumstances a double vein. More rarely pebbles and 

 other marks of water action are stated to occur in soft veins. 



Mode of Aggregation of the Ingredients. A variety of appear- 

 ances deserve special notice as indicating some of the conditions under 

 which the vein was filled. In some cases, for instance, the whole 

 breadth of the vein is occupied by one kind of substance, as lead ore, 

 or quartz, or sulphate of baryta; in other instances, the metallic 

 matter is interspersed in small masses through a basis, such as quartz ; 

 but, generally, the different substances which fill the vein are ranged 

 in a definite order of succession from the sides of the vein toward the 

 middle, in which, commonly, the metallic matter occurs in an irregular 

 vertical table, called a rib of ore. These conditions are best observed 

 in the proper veins, but are also to be noticed in the nests and detached 

 masses of ore and vein-stuff which sometimes occur in the vicinity of 

 the veins. 



General Idea of a Mineral Vein. The ordinary conception of a 

 mineral vein is well exemplified in some Derbyshire specimens, not 

 rare in collections, which, when cut across, show, in 

 the middle, masses or a continuous rib of galena, and b c a cb 

 on each side of this, to the extreme edges of the 

 mass (or narrow vein), layers of fluor spar and car- 

 bonate of baryta in frequent alternation, all the 

 materials being crystallised together without leaving 

 any cavities, yet preserving their own character of 

 structure. 



Thus, in the diagram, a is the middle rib of 

 galena, b b, c c, the alternating bands of barytic spar and fluor spar ; 

 d d, the masses of rock which enclose the vein, are called the walls or 

 cheeks of the vein. 



Supposed Successive Deposition of the Substances. The con- 

 templation of these specimens seldom fails to impress upon the 

 mind a conviction that the several bands of mineral substances 

 were deposited on the cheeks or walls of the vein in succession, the 

 middle being filled last of all ; and this theoretical notion has been 

 illustrated by comparing a mineral vein to a narrow gallery whose 

 walls were covered by many successive coats of plaster of different 



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