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MUSEUM OF COMPAIIATIVE ZOOLOGY. 



129 



1. Mclaphyrs of an aniygdaloidal character, known as "amygdaloid" 

 mines, which have been subjected to hot-water action, and whose depos- 

 its of copper are '' bunchy " and irrcguLir. These are in no sense veins 

 or lodes, and the Qiiincy and the Sheldon and Columbian mines are 



good examples. 



2. Ash-bed. mines, which are truly melaphyr or *' amygdaloid" ones, 

 but the melaphyr is of a more scoriaccous character, as was pointed 

 out above, of which the Copper Falls (in part) and the Atlantic mines 



arc examples. 



3. The conglomerate or true bed mines, hke the Calumet and llecla. 



4. The true fissure vein mines, iike the Central, Phcenix, and Copper 

 Falls (in part). 



The first two forms should be classed as one. 



It is an established rule that a mineral vein, in passing from one bed 

 of rock into another of a different nature, is apt to vary in width and con- 

 tents. Such variations in dimensions and gangue have been repeatedly 

 found in this district, as the different beds arc comparatively narrow. 

 As might be expected, the variation is not so strongly marked in the 

 dillerent mclaphyrs, since they are all rocks of the same composition and 

 origin, dillering only in texture, thickness, and crystallization. One and 

 the same vein may then vary in width, from a mere scam to thirty or 

 more foot, as is the case with the "Owd Creek Vein," as mined at Cop- 

 per Falls. Under tliesc conditions one cannot judge with any certainty, 

 from the appearance of tlio vein at one point, what will be its width or 

 character at another. The decision must be based on probabilities only. 

 In places the fissure may be filled with true vein matei'ial, while in other 

 parts, especially in the wider portions, it may be filled with fragments 

 of the melivphyr, cemented together by vein matter. The width of the 

 vein will depend largely upon the readiness with wdiich the country rock 

 yielded to the crushing and grinding force, and to the action of the per- 

 colating waters. We should expect, then, the veins to be mere nominal 

 fissures in the sandstones, conglomerates, and heavy "greenstone," but 

 to be more or less well marked in the "amygdaloids " and "traps." 



The filling of the veins and of tlie cavities in the melaphyi's, it ap- 

 pears, was accomplished by the same agencies. The amygdaloidal struc- 

 ture of the melapliyr is owing to the filling with mineral matter, with 

 ^a-eater or less completeness, ga.s cavities formed at the time of the lava 

 flow. Besides the true amygdaloidal structure, there arc numerous 

 cases of pseudo-amygdaloidal structure. I'liis last arises from the alter- 

 ation of the formerly solid parts of the melaphyr, and is to be found not 



VOL. VII. — KO. 1. 9 





