1212 A TREATISE ON METAMORPH1SM. 



found in pitching troughs underlain by relatively impervious strata ; and 

 ore deposits produced by ascending waters are often found in pitching 

 arches overlain by impervious strata. 



If this statement be reversed we have the suggestion that where ore 

 deposits occur in connection with pitching anticlines and synclines, their 

 positions furnish a criterion by which it may be decided whether their first 

 concentration was accomplished by ascending or descending waters. Where 

 the ores occur in pitching arches bounded above by impervious strata, the 

 presumption is that they were concentrated by ascending waters; where the 

 ore deposits occur in pitching troughs bottomed by impervious strata, the 

 inference is that they were concentrated by descending waters, for it is dif- 

 ficult to see how waters can be converged at such positions by moving in 

 the reverse directions. Of course this criterion can not be too rigidly applied, 

 for independently of the impervious strata, openings which so frequently 

 occur on anticlines and synclines might furnish trunk channels which could 

 be taken advantage of by ascending or descending waters. 



The Lake Superior iron ores furnish an admirable illustration of the 

 concentration of ores by descending waters in pitching troughs which are 

 on impervious basements. Since these ore deposits, which fully illustrate 

 the principles of concentration of ores by descending water in pitching' 

 impervious troughs, have already been discussed, ores of this class will not 

 be here further considered. (See PL XIII.) 



A case in which ore is probably deposited by ascending waters in 

 arches, because there concentrated by impervious roofs, is furnished by the 

 Bendigo gold district of Australia. a The typical position for the gold in 

 the district, according to Packard, is immediately below a slate, on top of 

 a sandstone. The slate is the impervious stratum and the sandstone the 

 pervious stratum. In this district there are a large number of alternations 

 of pervious and impervious strata, as a result of which concentrations have 

 occurred at a number of horizons, one above the other. While Packard does 

 not specifically speak of the pitch of the anticlines, the longitudinal sections 

 show that they do have a marked pitch. 



It is not supposed that the location of ore deposits in pitching arches 

 below impervious strata is wholly controlled by the existence of the impervi- 

 ous roofs, for, as already explained, when a heterogeneous mass of strata is 



a Rickard, T. A., The Bendigo gold field: Trans. Am. Inst. Min. Eng., vol. 20, 1892, pp. 463-545. 



