400 CONDITIONS OF MINERAL MATTERS IN VEINS. 



the distinction between rock dykes and mineral veins is imaginary, 

 they are in general clearly contrasted by the nature of the substances 

 which they contain. In dykes the crystallised minerals are of the 

 same kind as those great interior masses of consolidated rock from 

 which they often are evidently ramifications. In veins metallic sub- 

 stances occur which are mostly not known to exist in nature except in 

 these situations, and in others very similar or distinctly related to 

 them by position and minerals. To this general rule quartz is one of 

 the most striking exceptions ; yet even in this instance it is remark- 

 able that the quartz of veins is of a different aspect from that mingled 

 with the ingredients of granitic rocks. We must therefore take the 

 presence of metallic matter and certain non-metallic substances usually 

 connected therewith, and commonly called vein-stuff, as the leading 

 characteristic of the mineral veins, whose history we are now to 

 examine. 



Substances in the Veins. The simple minerals which occur in 

 veins and analogous situations are far more numerous than those which 

 are found as component parts of rocks. Igneous rocks, and especially 

 those of modern volcanic origin, hold a very great variety of non- 

 metallic substances, some of which also occur in veins; but it is 

 almost exclusively in veins that we find the metals in their pure state, 

 or alloyed with one another, or mineralised by combination with 

 sulphur, oxygen, chlorine, &c., or converted into salts by union with 

 various acids. Every elementary substance yet discovered by chemists 

 exists in the earth ; and it is probable that none of these are entirely 

 absent from the solid contents of mineral veins. 



Alloys. The metallic substances seldom occur pure. Sometimes 

 they are found in alloys, similar for the most part to those now pro- 

 ducible by the chemist, thus silver, antimony, cobalt, nickel, iron, are 

 alloyed with arsenic ; silver and nickel with antimony ; lead, gold, 

 silver, and bismuth with tellurium ; silver with mercury ; platinum 

 with gold, &c. The only known circumstance which stands as ante- 

 cedent to the production of such alloys is heat, produced by either 

 chemical action or pressure; and perhaps there is no single fact con- 

 nected with the theory of veins on which a belief in the influence of 

 heat in their production might be more securely based. 



Oxides and Salts. The metallic oxides prevalent in veins are 

 produced under various relations to heat, moisture, and contact with 

 gaseous substances ; and have various degrees of permanence when 

 exposed to high temperatures, either separately or combined. Metallic 

 salts are not rare in veins, and in the same way vary in their origin 

 and degree of permanence. Metallic oxides and salts, in very many 

 instances, are derivative compounds from sulphides and other primary 

 combinations. This is frequently the case with oxide of iron, car- 

 bonate of copper, and probably carbonate, phosphate, and other salts 

 of lead. 



Non-Metallic Minerals. Besides the metallic ores which impart 

 to many veins their most striking, if not most constant characters, 

 various earthy minerals lie in these repositories, and, as will afterwards 



