320 Professor John W. Gregory [April 27, 



would make the softer finer material flow into the interspaces 

 between the harder fragments ; the very force that fractured the 

 rocks would also compress them, so as to render the crushed mass 

 impermeable. Solutions might work their way along fractures, de- 

 positing ordinary fissure-veins, and replacement-veins on their edges ; 

 but there will be nothing like the widespread permeation which has 

 produced the great pyritic ore-bodies. It is only natural that in 

 these cases the mines should not reach the same depths as ores 

 deposited along the channels of solutions escaping from plutonic 

 depths. 



Shallow and Deep Ore Types. 



The distribution of ores in depth may, therefore, be usually 

 inferred from a combined study of the geological structure of a mining 

 field and the mineralogical characters of its ores. A definite conclu- 

 sion is not always possible, as in the absence of these data, or when 

 dealing with new types of ore-deposits. But in ordinary cases a 

 general opinion can be expressed. 



Thus there are three groups of ore-deposits which must ])e ex- 

 pected to have comparatively shallow limits. They are : — 



1. Ores that have been deposited in fissures and open spaces, 

 which cannot remain open at great depths. Hence, such ores if 

 continued deeper, will be continued by ores of different character. 



2. Ore-masses formed by the replacement of blocks of shattered 

 rocks, because masses of crushed rocks at great depths would be closed 

 by the flowing of the softer materials, and thus be impermeable to all 

 ore-bearing solutions. 



8. Ores formed by precipitation from descending solutions, includ- 

 ing nuggets, bonanzas, and the various types of secondary enrichments, 

 and also many ironstone masses and beds. 



On the other hand there are three types of ore-deposits which may 

 persist to great depths. They are : — 



1. Lodes characterised mineralogically by low-grade alloys, and 

 complex sulphides with small percentages of the more valuable metals, 

 and geologically by their occurrence as replacement veins along great 

 fault planes, or as isolated bodies along lines of special permeability 

 in folded rocks. 



The limit in depth of such ores is the level of maximum satura- 

 tion, which must occur as a rule at the depth of about l.s,()0() feet, 

 for at that depth the ascending plutonic waters must begin to deposit 

 their metallic salts. 



2. A second group of ores with a possible great extension in depth 

 are those of sedimentary origin, in tilted rock masses ; for they can 

 extend as deeply as such rocks can be buried without being meta- 

 morphosed by the earth's internal heat. 



?). The third group of possibly deep seated ores are those formed by 

 the segregation of metallic minerals from molten rock magmas. 



