362 ORIGIN AND DEVELOPMENT OE ORE-DEPOSITS. 



4. That tlie subterranean circulation which. I have 

 endeavoured to illustrate, aided and supplemented by electrical 

 transference and re-arrangement, by chemical re-actions going 

 on within the deposits and by the forces of crystallization and 

 its allies, will account for all the phenomena of lode filling. 



Sec. 12. — Theories of Impregnation. 



In the Presidential address of Prof. Smyth quoted in the 

 last section, reference is made to the "feeders" observed in 

 connection with many lodes, and to the frequently associated 

 mineralized "country" rocks, in the following terms: — "When 

 in the neighbourhood of a vein you find strings and specks of 

 some of the ores which it contains in bulk, some miners will not 

 hesitate to look upon them as " feeders " or contributories to the 

 vein which come in from the "country." Others will rather 

 look upon them as impregnations from the lode."* 



It might seem at first a mere problem of chemical analysis 

 to settle this point, but a little consideration will show that this 

 is not the case. I cannot do better than quote here the words 

 of Dr. Henry Wadsworth of Massachusetts, who observes 

 " since ore-deposits are generally associated with altered or 

 metamorphosed rocks, and occur in regions in which thermal 

 waters have been active, the country rock would naturally be 

 more or less charged, and sometimes completely decomposed. 

 In the process of the formation of the ore-deposit it may 

 happen that the ore-material will be entirely removed from the 

 adjacent rocks {i.e. to form the deposit in question), or this rock 

 may have deposited in it ores which never existed there before, 

 or, again, the ore-material may have been brought from a 

 distance by the percolating waters. Prom the above it follows 

 that chemical analyses alone, either of the country rock or of 

 its enclosed minerals, lead to unreliable conclusions as to the 

 source of the ores, and hence it is unphilosophical to build any 

 general theory upon such analyses."! 



Clearly then it is not always easy — or even possible — to 

 decide even in the simplest case whether the country rock 



*0p. cii.,p. 90. 



t Wadsworth. " Theories of Ore Deposit." Proc. Bost. Soc. Nat. Hist.,, 

 May, 1884, p. 201. 



