INFLUENCE OF IMPERVIOUS STRATA. 1207 



formed along trunk channels occur commonly, if not universally, to some 

 extent in preexisting openings and to some extent as a substitution for the 

 wall rock. Where the trunk channels are simple and the rocks refractory, 

 the ore deposits may be largely in preexisting openings. Where the trunk 

 channels are complex and the rocks soluble, the ore deposits to a large 

 extent are likely to be replacements. 



IMPERVIOUS STRATA AT VARIOUS DEPTHS. 



Slichter's theoretical investigations on the motions of ground waters 

 show that, in order to discuss the flowage under any given set of condi- 

 tions, it must be assumed that it is limited only by an impervious stratum." 



It is, of course, understood that there are no strata which are 

 absolutely impervious, but there are many which are practically so. 

 Wherever there is an impervious stratum the effective underground circula- 

 tion is there limited or divided, whether the stratum be at the depth of 100 

 or 1,000 or more meters. The impervious stratum may be a plastic shale 

 which yields to deformation without fracture, or it may be a rock intruded 

 after deformation has occurred, thus making a barrier. Of course there 

 are all gradations, from practically impervious strata to strata which merely 

 check the circulation. It is believed that in the average case the limit of 

 effective circulation is probably much less than the theoretical limit of 

 10,000 meters given by the depth of the zone of fracture. 



If an impervious stratum be but 100 meters from the surface and fissures 

 be limited to that depth or interrupted, the laws given (pp. 1021-1028) 

 apply to the circulation above the stratum. Therefore, such a fissure may 

 be occupied by ascending water in the lower part and by descending water 

 in its upper part. Hence an ore deposit contained in such a shallow fissure 

 may be the result of a single concentration by ascending' or descending 

 waters, or of two concentrations, the first by ascending and the second by 

 descending waters. 



The foregoing statement in reference to the practical limits of under- 

 ground circulation for the ore deposits of a given district may be true even 

 if below the impervious stratum there are other strata, fed from a distance, 

 in which circulation is occurring. Such lower pervious strata may have 



"Slichter, C. S., Theoretical investigation of the motion of ground waters: Nineteenth Ami . 

 Eept. IT. S. Geol. Survey, pt. 2, 1899, 329-357. . 



