THE iRON-BEAlimti MEMIVinj. "249 



less extent, rearrtuiged. This is indicated by tlie tacts that tlic silica is 

 quite completely crystallized, and that the siHca belts, instead of ))eing 

 laminated witli the carbonate, at times break across them in the most irreg- 

 ular manner. This fissuriug must have been siibse(|Ui-nt to the deposition 

 of tlie carboiiMte, and niay imply the entrance of silica from ;ni exti'uneous 

 source, but :dso niay mean no more tlian tliat silica ori^inal]\' present has 

 been taken into solution and recrystallized. In either case there is no proof 

 that the greater part of the silica was not an original deposition. 



It is assumed that such a cherty carbonate is water-deposited, as a 

 direct eruptive origin has never been maintained for a rock of this char- 

 acter. Such ;in origin has, liowever, been asserted for some of the ferru- 

 ginous cherts iuid jaspers which in other parts of the lake Superior country 

 are found associated with the carbonates.' 



Taking it for granted, then, that this cherty carbonate is a water- 

 deposited sediment, tlie questions arise, in what manner the iron carbonate 

 and siHca were originally dissolved and how they were precipitated. We 

 may, without varying too far from the law of unifol•mit^', l)elie\'e that in 

 very ancient times the atmosphere was more highly charged witli carbon 

 dioxide than at present. We may also believe tluit the rocks composing the 

 crust of tlie eai'th were then at a somewliat higher temperature. An increase 

 in heat oi' but m few degrees would be a powerful assistance in tlie process 

 of soljition of the ii'on, and this wonhl be especially true if the atmosphere 

 at this time was still richly charged with carVxin dioxide. The atmospheric 

 waters would absorb this acid and carr\' it into the rocks wliich l)or(lered the 

 ancient sea, would deconqjose them, and take in solution ferrous carbonate. 

 Such waters escaping into the shallow ocean at hand would hear the material 

 for these irony deposits. While it is believed that these conditions may 

 possibly have been present, their assunq)tion is not necessary to account 

 for the solution of the iron; for it is well known that thick beds of iron ore 

 have formed in recent times b\- virtue of the solubilit\' of iron as a car- 

 bonate under ordinary conditions. It is ,also not impossible that its solu- 



' Notes on the Geology of the Iron and Copper Districts of lake .Superior, by M. E. Wadsworth. 

 Bull. Mus. Comp. Zool. Harvard Coll., whole series, vol. vii, Geological series, vol. 1, No. 1, 

 1880, pp. 62-68. 



