LAWS AFFECTING METALLIFEROUS VEINS. 241 



in the stream-beds will be found gravel- drifts, composed partly of the 

 country rock and partly of the quartz vein-stone. Among the gravel 

 will be found particles of gold, washed out from the upper parts of the 

 veins. By the sorting power of water the heavy gold particles are apt 

 to accumulate mostly near the bed of the gravel-deposit (bed-rock). 

 These gravel-deposits are the placers. In these, the gold-particles, like 

 the stone-fragments, are always rounded and worn by attrition. 



Some Important Laws affecting the Occurrence and the Richness of 

 Metalliferous Veins. 



1. Metalliferous veins occur mostly in disturbed and highly-meta- 

 morphic regions, where the strata are tilted, and folded, and metamor- 

 phosed. The tilting and folding are necessary to the formation of fis- 

 sures ; and the conditions under which metamorphisni takes place seem 

 necessary for the subsequent filling with mineral matter. Mineral veins, 

 therefore, occur mostly in mountain regions, and in the vicinity of more 

 or less obvious evidences of igneous agency. Lead- veins seem to be an 

 exception to this rule. They are often found in undisturbed regions 

 where the rocks are entirely unchanged. The rich lead -mines of Illi- 

 nois, Iowa, and Missouri, are notable examples, the country rock being 

 horizontal, fossiliferous limestones of the Palaeozoic era. 



2. Metalliferous veins occur mostly in the older rocks. In Great 

 Britain, for example, no profitable veins occur above the Trias. This 

 rule, which was regarded as of great importance by the older geologists, 

 is not so regarded now. There seems to be no close connection between 

 the occurrence of metalliferous veins and simple age alone ; the con- 

 nection is rather with metamorphisni. Metamorphism, as we have seen, 

 (p. 219), is most common in the older rocks, and becomes more and 

 more exceptional as we pass upward. The occurrence of metalliferous 

 veins follows the same law. But when the newer rocks are metamor- 

 phic, they are as likely to contain veins as are rocks of the older series. 

 The metalliferous veins of California occur in Jurassic, Cretaceous, and 

 even Tertiary strata ; but these strata are there highly metamorphic, and 

 strongly folded. In Bohemia, also, and elsewhere, metalliferous veins 

 occur in the higher series (Phillips's Geology, p. 549). 



3. Parallel veins are apt to have similar metallic contents, while veins 

 running in different directions (unless sometimes at right angles) are apt 

 to contain different metallic contents. Thus, the nearly east-and-west 

 lodes of Cornwall, a a a and c c (Fig. 217), contain tin and copper, 

 while the north-and-south courses, b b, contain lead and iron. The au- 

 riferous veins of California are parallel to each other and to the Sier- 

 ras, except a few smaller ones, which are at right angles to these. The 

 reason of this rule is, that parallel fissures belong to the same system, 

 and were therefore formed at the same time, broke through the same 



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