1216 A TREATISE ON METAMORPHISM. 



dike cuts across the stratum, a pitching' trough may be produced, as, for 

 instance, in the Penokee-Gogebic district of Michigan and Wisconsin." An 

 intruded igneous rock may follow the contact between folded strata and 

 thus furnish a trough or arch bounded by an impervious formation. Various 

 other ways will occur to one in which pitching troughs or arches with 

 impervious basements, or roofs, or both, may be produced. A trough or 

 arch, no matter how formed, is favorable to the concentration of ores, pro- 

 vided a porous stratum or an opening between the layers furnishes a trunk 

 channel. Of course, other favorable conditions must cooperate with these 

 in order to produce an ore deposit. 



Combinations of pervious and impervious strata, united with joints, 

 faults, and other structures which affect some impervious strata and not 

 others, furnish extraordinarily complex sets of conditions which will 

 undoubtedly yield interesting results when studied in special cases. 



GENERAL STATEMENTS. 



The importance of variation in porosity and structure is sufficiently 

 illustrated in the foregoing- pages to show that one of the first steps in study- 

 ing the genesis of ore deposits formed by aqueous solutions is to investigate 

 the underground circulation. If a geologist wishes to know whether or 

 not adequate supplies of underground water can be obtained for a city, he 

 should at once begin to study the strata, in order to ascertain their varying 

 porosity, their accessibility, the amount of water they carry, their import- 

 ance as water bearers and water restrainers, and the possible sources 

 of supply: in short, he should make a complete investigation of all of the 

 factors which influence the underground circulation for that district. Fre- 

 quently this would involve the study of the stratigraphy for extensive areas, 

 often for entire geological provinces. From my own point of view it is 

 equally important, in order to ascertain successfully the genesis of the ore 

 deposits of a given district, to begin by studying- the stratigraphy in all its 

 bearings, especially with reference to underground systems of circulation. 

 The problems of stratigraphy and underground circulation being solved, 

 including both the ascending and descending currents, the geologist will 

 have gone far toward the solution of the problem of the genesis of the ores 



"Irving, R. D., and Van Hise, C. B., The Penokee iron-bearing series of Michigan and Wiscon- 

 sin: Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 268-275. 



