102 V "-VEINS THEIR NATURE AND ORIGIN. 



-or- Tii&aing /parallel to each, other according to the most 

 prevalent direction of the earthquake convulsions. These 

 fractures will vary, of course, from mere cracks to yawning 

 chasms many feet in width, and will descend at all inclina- 

 tions some sloping downwards at a low angle, and others 

 sinking perpendicularly, or nearly so. Now it requires no 

 great effort of the understanding to perceive that these 

 fissures, in course of time, will be filled either by matter 

 washed in from the earth's surface, or by volcanic and 

 mineral substances injected from the interior. Such rents 

 in the stratified rocks, when thus filled up by lava, by 

 greenstone, or by basalt, are generally known as " dykes," 

 the igneous matter rising up like a wall through the strata 

 on either side. On the other hand, when they are filled 

 by sparry or crystalline minerals, and these intermingled 

 more or less with metallic ores the slow and gradual de- 

 positions of chemical agency they are usually distinguished 

 as " Veins," from their traversing and ramifying through 

 the crust like the veins through the animal system. But 

 the veins that were formed at one period may be cut 

 through or crossed by others of a later era, and thus in 

 many districts there is a network, as it were, of veins, 

 crossing and intercrossing in a very complicated manner. 

 As might be expected, too, the original veins may contain 

 one kind of mineral or metal, and the cross- veins another 

 kind, and hence the greater richness, as well as complexity, 

 of many metalliferous regions. 



Understanding, then, that " dykes " consist of unproduc- 

 tive rock-matter, and that " veins " are always less or more 

 metalliferous, it may be stated as a fact, that the latter 

 occur most abundantly, and naturally so, in the primary 

 formations. These are the rocks that have suffered most 

 from igneous convulsions, and these also are the rocks 

 among which metamorphism and chemical agency have had 



