394 



THE GEOLOGIST. 



question is tlie first difficulty we have to grapple with, and it has 

 been fruitful of hypotheses. The Wernerian theory that the contents 

 of veins (the most common form of metalliferous deposit) were de- 

 rived from above has been universally abandoned. There remain 

 two others : one, that these deposits originate by segregation from 

 the neighbouring rock ; and the other, that they are derived, by some 

 means or other, from beneath. The segregation theory is one well 

 worthy of consideration, and undoubtedly applies to many metal- 

 liferous deposits ; but it still leaves unsolved the main problem of the 

 original source of the metals ; for it completely fails to account for 

 the abundant distribution of the metallic ores in some districts, while 

 in others, in the same class of rock, they are entirely absent. To ac- 

 count for all metalliferous deposits by segregation, irrespective of any 

 other cause, we should have to have recourse to the alchemic doctrine 

 of the possible transmutation of earths into metals. The theory of 

 the metalhc ores from beneath, has been suggested in every form ; 

 and is not without many difficulties to which I shall refer farther on. 



III. Various forms of metalliferous dejposits. — Next to the question 

 of the metallic ores, we are met with difficulties depending on the 

 forms in which they are usually found. They generally occur in 

 abundance in veins of a definite size and direction — the direction being 

 constantly associated, in certain districts, with characteristic ores. 

 But they are also found in forms that are not veins, such as in 

 " stocks," or in irregular and indefinite masses. Veins are not 

 peculiar to ore-formations, but are found in almost every rock, and 

 include every kind of mineral species. Still, as from their nature, 

 veins are hidden far from our sight ; we only become intimately ac- 

 quainted with them when they consist of such minerals that the 

 necessities of man lead to their exploration. Hence our knowledge 

 of veins is principally confined to those containing ores, or other veins 

 associated with them; and we are often consequently led, very 

 erroneously, to imagine that no veins exist but ore-veins, or those 

 connected with them, and to lunit the occurrence of minerals in the 

 peculiar /o^rn we denominate veins exclusively to minerals of the use- 

 ful metals : an idea productive of considerable misapprehension and 

 confasion. 



ly. Most mineral-veins tvere fissures, suhsecptently filled. — This is 



