THE IRON BEAEING MEMBER. 255 



and thus concentrations of the iron oxide in Vjands and shots would occur. 

 This explanation is not a wholly suppositious one, but accords with the facts 

 of observation as seen in numerous exposures, one of which, the cliff bor- 

 dering' the outlet of Sunday lake, in Sec. 13, T. 47 N., R. 46 W., Michigan, 

 is fully described (section iv) in connection with the origin of the ores. 

 The concentration of silica in these cherts is partly explained by the 

 abstraction of ii'on carbonate taken into solution. This chert is rarely pure, 

 and frequently contains a large amount of iron oxide; and this is what 

 would lie expected from the character of the processes above indicated, yet 

 the silica is much more abundant in the ferruginous cherts than in the unal- 

 tered carbonates. It is to be remembered that most of these cherts are at 

 low horizons, and it is probable that they have to a considerable extent 

 been silicitied. The proof of this silicification lies both in tlie iiliundance 

 of the silica itself and in the manner of its occurrence. The greater part 

 of it is relatively perfectly crystalline as com{)ared with the chert of the 

 carbonates. The cavities which the rock frei|uently contain are lined with 

 quartz crystals. Numerous veins of quartz cut through the rock in every 

 direction. The concretions, as explained (pp. 205-20U), are areas which 

 were originally carbonate of iron, but are now largely com[)oscd of 

 silica. Many of them have been sevei'ed by veinlets of silica. All these 

 facts imply an extensive rearrangement of the silica originally present and 

 the introduction of additional silica. In a subscipient consideration of the 

 origin of the ore deposits it will be seen that it is proliable that the silica 

 necessary for this silicitication was derived from original chert}' curlionates 

 which have been swept away by erosion. If this is the case, a jxirtiou of 

 the silica within the ferruginous cherts must have been carried some 

 distance. The ferruginous cherts are cut, as shown bv mining operations, 

 by luunerous dikes, which are much decom[)osed and ai-e certainly more 

 basic than they were originally. This alteration of the dikes may ;dso 

 have furnished a portion of the silica for the silicitication of these ferrugi- 

 nous cherts. The nature of the solutions which dissolve silica in rocks and 

 deposit it in other places is becoming better kiiown, and that it is hirgely 

 carried is t-H-rtain/ The quantity of silica re([uired to till the interspaces of 



' For foreign localities, see Roth's AUgcuiciue niid Chemi.sclie Geologi.-,, vol. I, 1879. For 

 United States localities, see Bulletins of the II. S. Geological Survey, No. 32, List aufl Analyses of the 



