1202 A TREATISE ON METAMORPHISM. 



COMPLEXITY OF OPENINGS. 



In the general discussion an ore deposit has been spoken of as if it were 

 a single continuous mass formed in a large opening. An ore deposit in a 

 single large opening is exceptional. Ore deposits show all gradations of 

 openings from large single ones to those of an extraordinarily complex 

 character. A trunk channel maj^ be a set of distributive faults ; a group of 

 parallel or intersecting- sets of joint openings; the minute parallel openings 

 of fissility; a group of openings along bedding planes; the shrinkage open- 

 ings formed within or along the borders of cooling magma; the openings in 

 an autoclastic rock or reibungs breccia; the multitude of openings of a 

 sandstone or a conglomerate; or the very minute openings between the 

 mineral particles of a limestone, schist, gneiss, or other dense rocks. 



Trunk channels may vary from vertical to nearly horizontal attitudes. 

 Ore deposits ordinarily have important vertical components, although they 

 may be found in nearly horizontal positions. In the latter case the trunk 

 channels forming the deposits probably have vertical components some- 

 where else. 



It is hardly necessary to give illustrations of ore deposits for each of 

 these complex conditions, but, as very excellent instances of veins of a very 

 composite character may be mentioned the Cripple Creek deposits a and the 

 gold-quartz veins of Nevada City and- Grass Valley, Cal. 6 The essen- 

 tial point, so far as~ the discussion of the foregoing pages is concerned, 

 is that ore deposits commonly occur at places where there are trunk chan- 

 nels for ascending or descending waters, or both. In order that metallifer- 

 ous material shall be brought to a place and deposited in large quantity, 

 there must be long-continued circulation. It matters not whether a trunk 

 channel is a single passage or is composed of an indefinite number of minor 

 passages, the principles given on the previous pages are applicable to the 

 deposition of ores in such trunk channels. 



In various regions the conditions are so exceedingly complex that ore 

 deposits close together may differ greatly in their mineral content. 

 This is the best evidence that, notwithstanding their continuity, the under- 



a Penrose, R. A. F., jr., Mining geology of the Cripple Creek district, Colo.: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1895, pp. 137-153. 



SLindgren, Waldemar, The gold-quartz veins of Nevada City and Grass Valley, California : Seven- 

 teenth Ann. Rept. TJ. S. Geol. Survey, pt. 2, 1896, pp. 158-160, 259. 



