242 



STRUCTURE COMMON TO ALL ROCKS. 



strata, and were filled under similar conditions, and therefore with the 

 same materials ; while fissures running in different directions (unless in 

 some cases at right angles, p. 228) were probably formed at different 



Fig. 217.— Map of Cornwall: a and c, tin and copper; b, lead and iron. 



times, broke through different strata, and were filled under different 

 conditions. Thus, the east-and-west veins of Cornwall, a a, are pre- 

 triassic ; the north-east and south-west veins, c c, break through the 

 Trias, and are therefore post-triassic,* while the north-and-south veins 

 break through the Cretaceous. The auriferous veins of California all 

 break through the Jurassic ; the} T , or their fissures, were probably pro- 

 duced at the same time, viz., at the time of pushing up of the Sierras. 



4. A change of country rock of an outcropping vein is apt to deter- 

 mine some change, either in the contents or in the richness of the vein. 

 Nevertheless, there is not that close connection between the nature of 

 the country rock and the vein-contents which obtains in infiltrative 

 veins. The reason is, that infiltrative veins derive their contents entirely 

 from the wall-rock on either side, while fissure- veins derive their contents 

 from all the strata through which they break, even to great depths, and 

 especially from the deeper strata. The nature of the surface or country 

 rock is, therefore, only one factor, determining the vein-contents. 



5. Metallic veins are usually richer near their point of intersection 

 with granite or with an igneous dike, especially if the strata have 

 suffered metamorphism. This shows the influence of such heat as is 

 present in metamorphism, in determining the metallic contents. 



6. If two veins cross each other, especially if at small angle, one or 

 both are apt to be richer at the point of crossing. No sufficient reason 

 has been given for this law. It is probably due to the reaction of 

 waters bearing different materials circulating in the two fissures. 



* De la Beche, Geological Observer, p. 757. 



