PREEXISTING CHANNELS AND REPLACEMENTS. 1205 



the metallic copper. While the conglomerate deposits of Lake Superior 

 are in part replacements, they also are in large part fillings of preexisting 

 cavities between the clastic particles. An excellent example of replacement 

 in igneous rocks where there is wonderfully complex distributive faulting 

 and folding, and thus a large surface of contact for substitution, is furnished 

 by the Cripple Creek district, in which, according to Penrose," ore mainly 

 occurs replacing and blending into various igneous rocks. 



By substitution a rock may be completely replaced by ore. . This is 

 particularly likely to occur where the rock is uniform in structure and com- 

 position, as limestone or dolomite. Where, on the contrary,, the rock is of 

 complex composition, such as granite or porphyry, or where there are 

 different kinds of rock present, as, for instance, diorite and granite, the 

 replacement is usually largely selective. This selective replacement may 

 apply to the mass of the wall rock, to the individual fragments of it, to 

 clastic fragments of sandstones or conglomerate, and to the different con- 

 stituent minerals in a single fragment. The particular minerals or masses 

 which are most soluble in the solutions present are most rapidly dissolved. 

 Where the wall rock varies greatly in the solubility of its minerals the 

 selective replacement of the country rock may extend for some distance 

 from the central deposits. The readily soluble minerals are dissolved, and 

 in place of them are precipitated the metalliferous minerals. This process 

 is ordinarily called impregnation. Selective replacement of this kind is 

 well illustrated by the Butte, Mont, granite, in which "the basic constitu- 

 ents of the granite are naturally attacked first, then the feldspars, and 

 finally the quartz itself may be removed, so that in some parts there are 

 found large masses composed entirely of metallic minerals." 6 



In regions of heterogeneous rocks the frequent occurrence of main 

 masses of the ore deposits in the more soluble rocks is clue in part to its 

 greater solubility. For instance, where limestone occurs in intimate asso- 

 ciation with sandstone, with quartzite, with diorite, with trachyte, with 

 porphyry, with granite, or with almost any other rock, and ore deposits are 

 found, the ore is likely to be largely in the limestone, partly because it is 



"Penrose, E. A. F., jr., The mining geology of Cripple Creek, Colorado: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1895, pp. 140-141, 144-146, 161-162. 



* Emmons, S. F., Notes on the geology of Butte, Montana: Trans. Am. Inst. Min. Eng., vol. 16, 

 1888, p. 57. 



