COMPLEXITY OF ORE DEPOSITS. 1223 



V. GENERAL STATEMENTS. 



It is clear from the foregoing that an ore deposit may have a wonder- 

 fully complex history. It may not represent the work of a single period of 

 ascending or of descending waters, or one cycle of ascending and one cycle 

 of descending waters, but may be the result of a number of cycles of depo- 

 sition by ascending or descending waters, or both. 



Thus irregularities in certain of the ore deposits in very ancient rocks 

 may be explained. But in many cases it appears probable that the main 

 work in an existing ore deposit has been done by continuous concentration, 

 first by ascending waters, second by descending waters, although in such 

 cases part or even all of the metals may have been derived from earlier 

 concentrations. 



Any of the special and local factors above discussed and others not 

 considered may in an individual case be so conspicuous as to appear to be a 

 controlling factor in the formation of an ore deposit. One might say that 

 the existence of a given trough was the cause of the production of an ore 

 deposit. The truer statement would be that the factor under consideration is 

 one essential among many. The porosity of a formation, the existence of a ' 

 pitching trough, favorable topography, the presence of igneous rocks fur- 

 nishing heat to make the waters active, and many other special factors may, 

 in a given case, all be essential, and without the help of any one of them an 

 ore deposit would not have been produced, but no combination of factors 

 will form an ore body if a source of the metal is not available upon which 

 the underground waters may act. In short, each case of the development 

 of an ore deposit requires the fortunate combination of many favorable 

 factors, working harmoniously together, the absence of any one of which 

 may prevent the concentration of the ore deposit. 



VI. ORE SHOOTS." 



No fact is better known concerning ore deposits than that they vary 

 in the most remarkable fashion, both in size and richness. Moreover, these 

 variations are both vertical and hori zontal. Frequently rich deposits decrease 

 in size or are wholly cut off with extraordinary abruptness. Other equally 

 rich deposits may appear somewhere else on the same level or on another 



a For a general discussion of ore shoots in fissures, see Penrose, R. A. F., jr., The mining geology 

 of the Cripple Creek district, Colorado: Sixteenth Ann. Sept. U. S. Geol. Survey, pt. 2, 1895, pp. 

 162-166. 



