MUSEUM OF COMPARATIVE ZOOLOGY. 129 



1. Melaphyrs of an amygdaloidal character, known as "amygdaloid" 

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

 its of copper are " bnnchy " and irreguhir. These are in no sense veins 

 or lodes, and the Quincy and the fcihelden and Columbian mines ai-e 

 good examples. 



2. Ash-bed mines, which are truly melajihyr or " amygdaloid " ones, 

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

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

 are examples. 



3. The conglomerate or true bed mines, like the Calumet and Hecla. 



4. The true fissure vein mines, like the Central, Phoenix, 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 are corapai'atively narrow. 

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

 difierent melaphyrs, since they are all rocks of the same composition and 

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

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

 more feet, as is the case with the " Owl Creek Vein," as mined at Cop- 

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

 from the appearance of the 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 material, while in other 

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

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

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

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

 colating w^aters. 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 the cavities in the melaphyrs, it ap- 

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

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

 greater or less completeness, gas cavities formed at the time of the lava 

 flow. Besides the true amygdaloidal structure, there are numerous 

 cases of pseudo-amygdaloidal strvicture. This last arises from the alter- 

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



VOL. VII. — xo. 1. 9 



