1204 A TREATISE ON METAMORPHISM. 



granite, and porphyry; and ore deposits which are largely replacements 

 are more likely to be found in easily soluble rocks, such as limestone. The 

 gold-quartz veins of California give an excellent illustration of the deposi- 

 tion of ores in preexisting cavities in refractory rocks, such as siliceous 

 argillite, diabase, diorite, and granodiorite. a This case is all the more 

 interesting since the wall rock itself is greatly modified, and has lost and 

 gained various elements. The San Juan district of southwestern Colorado 

 furnishes another excellent illustration of veins deposited in openings of 

 hard, refractory rocks. Here great continuous lodes in preexisting open- 

 ings are known to extend for a vertical distance of 1,000 meters or more; 

 but even here there is very frequently complex fracturing or even breccia- 

 tion, as a result of which the veins form either a series of sheets separated 

 by layers of wall rock or breccia cemented by vein material. Ore deposits 

 which are largely replacements in easily soluble rocks are well illustrated by 

 the silver-lead deposits of Eureka, Nev., 6 and Leadville" and Aspen, d Colo. 

 Replacements are likely to be important also in proportion as the 

 trunk channels are complex rather than simple. This follows from the law 

 of mass action. In proportion as a trunk channel is complex, the surface 

 of action upon the wall rock for a given quantity of solution is large. 

 As conspicuous examples where there are large surfaces of action may be 

 mentioned sandstones and conglomerates and the reibungs breccias, or 

 crushed rocks along fault zones. Where the trunk channels are very 

 complex, the rocks, even if refractory, may be replaced to a considerable 

 extent by the metalliferous ores. A conspicuous instance of this in a 

 sedimentary rock is that of the copper conglomerate deposits of Lake 

 Superior, where many grains, pebbles, and bowlders of porphyry are partly 

 or wholly replaced by metallic copper. On some bowlders the metallic 

 copper occurs as partial or complete skulls surrounding the fragments of 

 porphyry; in others these skulls are thicker, and in still others the entire 

 masses of the bowlders, as described by Pumpelly, e are fully replaced by 



«Lindgr.en, cit, pp. 172-257, 259, 261; also pp. 146-157. 



6 Curtis, J. S., Silver-lead deposits of Eureka, Nev.: Mori. U. S. Geol. Survey, vol. 7, 1884, pp. 

 93-106. 



c Emmons, S. F., Geology and mining industry of Leadville: Mon. U. S. Geol. Survey, vol. 12, 

 1886, pp. 565-569. 



^Spurr, J. E., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31, 

 1898, pp. 231-234. 



''Pumpelly, R., Copper district: Geol. of Michigan, vol. 1, pt. 2, 1873, pp. 37-38. Pumpelly, E., 

 The paragenesis and derivation of copper and its associates on Lake Superior: Am. Jour. Sci., 3d series, 

 vol. 2, 1871, pp. 348-355. 



