THE moy BEARING MEMBER. 285 



considering the nature of tlie belt of rofk ah»ove tlie ore formation. It is 

 a member composed of black and g-ray cla}- elates, graywafkeri, and grav- 

 wa<^-ke-slates ; all of whic-h contain a large amount of clay and are practi- 

 cally impervious to water. So stnmgly clayey is this formation, that a rock 

 underlying any great thickness of it couhl not be much affected by waters 

 from above. An independent proof of the inij^enetrable character of this 

 rock is the freshness of the c-ontained greenstones, which, as explained in 

 chapter vii are almost or wholly decomposed in the lower parts of the iron 

 formation. Mowever, when the series had been uplifted and errxled, this 

 upper imjiervious member would have Vjeen removed, and the waters would 

 come directly in contact with the lower horizons of the ore formation, 

 while the upjjer horizons of the belt would still be somewhat protected. 



Before gojng further it is necessary to consider the jjorosity of the 

 rocks which underlie the ore fonnation. This member has been fully 

 described (chap. iv). It consists maiidy <>f a layer of feldspathic quartz- 

 slates. These are interstratified with day slateg, consequently the rocks 

 of this fonnation are almo.st imj>enetrable to percolating waters. Mntve 

 the feldspathic quartz-slates, and therefore I>etween them and the ore, is a 

 layer, a few feet in thickness, of coarse fragmental «juartzite — a rock 

 which was once a samlstone. Before the induration of this .sandstone, j>er- 

 colating waters may have penetnited it, but they would have been stopped 

 in their downward passage by the underlying .slates. After its cliange to 

 a quartzite it was it.self a barrier to the passage of percolating waters. It 

 is, however, not so perfect a check as the underlying slates, because of the 

 fracturing to which it lias Ijeen subjected. The joints, so characteristic of 

 a brittle quartzite, do not affect the underlying slates, for these thinly lam- 

 inated clay rocks are flexiV>le, and under the slight Ixjwing which thev 

 have received are almost or quite as impervious to water a« when in their 

 horizontal |K>sition. 



Frocefif of concentrution. — An attempt will now be made t<j trace the 

 ])a."«sage of jjercolating waters through the inclined layers of the iron 

 fi»rmatiou. PI. xxxi, P"ig. 7, is a section showing the condition of this 

 meni^>er at the present time at the surface and illustrating how this state 

 of aiiairs was reached. The strata of the formation are now exjiosed. by 



