

MINERAL VEINS AND ORE-DEPOSITS. 777 



sitions of the fissure-like and cavernous openings were in part deter- 

 mined by the joints in the rocks. The ores of Eureka, Nevada (and 

 of some other western mines), are similar in occurrence. But, be- 

 sides lead, they yield gold and silver, and the chief ore is lead carbon- 

 ate. With this, and other lead salts, and much red iron oxyd, there is 

 some argentiferous galenite, and auriferous arsenopyrite — the min- 

 erals from whose oxydatiou, says Raymond, the existing products have 

 mainly come. 



The filling of the cavities and fissures of amygdaloids, or any igneous rock, has been 

 produced on the same general method with the filling of veins, although the species 

 transferred to the cavities are mostly different because of the peculiarities in compo- 

 sition of the rock adjoining the cavities. The banding of the agates of such cavities is 

 made on the same principle with the banding of a vein, namely: deposition against 

 the walls; and the agate has often a surface of quartz crystals at centre, as a band of 

 quartz in a vein has its comb of crystals pointed toward the middle of the fissure. 



The materials that fill the cavities (making the amygdules) and the fissures may for 

 the most part be traced directly to the adjoining rock. The elements are in most cases 

 the same, except that water is added — and this water is part of that which has made 

 the whole rock semi-metamorphic (p. 748). The chlorite, which is usually the first 

 lining of the cavities, has come chiefly from the pyroxene of the rock; the zeolites, from 

 the feldspars, from which they differ little in composition except in the presence of water ; 

 the silica, making the chalcedony, onyx, agate, carnelian, opal, hyalite, and quartz 

 crystals, of the cavities, is largely from the altered pyroxene, this mineral having 50 to 

 60 per cent, of silica, while chlorite has but 25 to 35 per cent. ; and the calcite, often 

 abundant, from the lime of the feldspar and pyroxene along with carbonic acid taken 

 into the rock during its eruption. The ingredients that are of outside origin are the 

 water and carbonic acid; and, sometimes, especially in the fissures, various ores, which, 

 if abundant, make them contact veins, as described beyond. The depositions were 

 made as the cooling of the solidified igneous rock went forward. Daubr^e has shown 

 that ehabazite and harmotome, as well as calcite, are now forming in crystals in the in- 

 terior of the bricks of Roman masonry at the warm springs of Plombieres, Bourbonne- 

 les-Rains, and Luxeuil, in France, and at Oran in Algiers ; at Oran the temperature is 

 not above 116|° F., and at Luxeuil, only 115° F. 



(3.) Dike-like Veins, — Whenever, in the progress of metamorphic 

 changes, a rock was rendered plastic, the material, whether granite, 

 limestone, or of other kinds, would have risen into all fissures that 

 were opened down to it. In this way veins of granite and of other 

 rocks have sometimes been made. When of granite, they differ from 

 veins of infiltration in being like ordinary granite in their uniform 

 grain and other characters, free from all banding and from coarse crys- 

 tallizations. The granite of such veins is often a good building stone, 

 while that of infiltration veins is too irregular in grain for such a use. 



Near the New London (Conn.) light-house, a fine grained whitish granite constitutes 

 veins of this kind coming up through gneiss, while at other places in that vicinity it is 

 in extensive beds so associated with the gneiss as to leave no doubt of its metamorphic 

 origin. Again, at Birmingham, Conn., where coarsely porphyritic gneiss and mica 

 schist occur in frequent alternation, a large vein of porphyritic granite, structureless, but 

 otherwise identical in aspect with the gneiss, at one place breaks through the mica 

 schist, and has fragments of the schist involved in it. In northern New York there are 

 veins of crystalline limestone of this kind, as described by Emmons. 



