VEINS. 407 



minolooy to express the i-el;itioiis of these deposits sncchictly and clearly. 

 Merely formal dassilication is of little value, but, since exhaustive descrip- 

 tions of structure and form cannot be given on every occasion, deposits must 

 in some way be referred to recognized types; and, unless such references are 

 generally understood in the same sense, statements involving them will nec- 

 essarily fail to convey the meaning intended. I am certain that the term 

 vein as used by miners and geologists embraces types of structure which, 

 though closely allied, differ greatly from one another, and I have found that 

 many of the discu.ssions which constantly arise as to whether a particular 

 deposit is or is not a vein and as to the limits of veins owe their origin to 

 the ambiguity of this term. My opinion as to the nature of the quicksilver 

 deposits of the I'acirtc Slope will perhaps be more readily intelligible if a 

 few paragraphs are devoted to the discussion of this subject. 



NATURE AND NOMENCLATURE OE VEINS. 



Fissures and cavities. — Exceptiug wlieu orcs are deposited in beds, like coal, 

 or in placers, like gold gravels, the existence of open subterranean spaces 

 of greater or less size is a necessary condition for the formation of ore 

 bodies of any kind. The ore may be deposited in openings which existed 

 before deposition l)egan and which either were cracks between masses of 

 rock bi-oken asunder or were interstices in porous rock, like sandstone. 

 Room for the ore may also be made by solution of the rock mass either 

 before ore deposition or during that process. It is a mistake, however, to 

 suppose that the masses stoped out in the exploitation of mines usually 

 i-epi-esent spaces which were empty before the deposition of ore in them. 

 It is said that cases occur in which open caves in limestone have been filled 

 with ore, but cavities of this kind appear to be confined to limestone and to 

 liave formed only above the water level of the district by the solvent action 

 of surface waters charged with carbonic acid Ore deposits, however, are 

 l)y no means confined to limestones and are more often found in rocks 

 of this class which have never been drained than in those wdiich have 

 been exposed under the conditions needful for the formation of caverns. 

 It is also conceivable that yawning fissures should form in the earth's crust 

 and tliat these should l)e filled up solely with ore and gangue minerals; but 



