248 THE PENOKEE' IRON-BEABING SE1ME8. 



recent times is in the clierty limestones, wliieh are so \videl\' found at all 

 horizons. It may be a ([uestion as to these limestones whether the chert was 

 originalh- in nodules and layers or was scattered through the limestone as 

 disseuiinated ])ai'ticles of siiica, but from later investiji'ations it appears certain 

 that iu some cases tlic cliert A\as originally de])osited in tlie jxtsition in ^\•hi(•h 

 it is now foinul. There is in these limestones a close association l)etweenthe 

 cherts and calcium and magnesium earbouates. In the chert}* iron carbon- 

 ates of the Penokee-Gogebic series calcium and magnesium are present as in 

 the limestones, and Ave need onlj' to re])lace a pi)rtion of these elements by 

 iron to luive rocks which are the exact analogue of the clierty limestones of 

 later times. As lias l>een seen also, such cherty limestones occur in deposits 

 of considerable thickness at the l)asc of tlie Penokee-Gogebic series itself. 

 From analogy it is then cxtremelv })robal)le that the chert and iron car- 

 bonates were sinudtaneoush' deposited, although it may he a possibility (as 

 has been maintained with reference to some of the cherty carbonates of later 

 time) that tlic chert eutei'ed very earl}" in the history of tlie rock as a 

 pseudomorph, replacing carbonates. 



At any rate, it is certain tliat a large portion of the silica now present 

 in these rocks was there \ery early in their history. In other districts 

 in the lake Superior countr\', notably at ({uiiflint lake, in tlie Animikie 

 series, a similar cherty carlioiiate is found in extensive lieds. "^Diat the 

 chert was here jiresent at a very early day can not be doiilited. The 

 chert\' bands, wheiv there is no folding, are eveiih' interlamiiiated with 

 the carbonates, but in folded areas tlie brittle cherts have been frac- 

 tured in every direction, so that ihe rock, instead of being a regularly 

 laminated one, is a breccia, which contains angular fragments of chert of 

 greatly varying sizes. Such a hrecciated rock fiv(|U(^iitly runs into the 

 regularly laminated kinds in the space of a few inches. It is tln^ii certain 

 that the chert of these beds was present l>efor(^ the folding of the rocks. 

 Further, there is no exideiice that most of this chert was not deposited 

 simnltaneousU' or alternatel\' with the iron carbonate. Tlie relations ot a 

 portion of the chert and the iron carlionate are, howcxci', such as to show 

 that either the chert has entered by subsequent solution, or that the silica 

 originally deposited with the carbonate was subsequently, to a greater or 



