tllK Il!UN-DEAl{lN(i MKMr.El?. 24? 



ences of opiniou as to the inauiu'r iii wliicli, tVoin the clicrtv carlionati s, 

 some of the multitiidinous pliases of rock now found liaxc hccii fdnucd; 

 but tlu' conclusion can not lie escaped that these rocks, wliich still make up 

 so hirge a proportion ol' the licit, were the orifjfiual rocks of tlic nicmlicr. 

 It is extremely improbable that, in a narrow belt of regularly stratified 

 rocks about 800 feet thick, interstratified with other Ixdts of sedimentary 

 rocks, there should be deposited in patches here and there, large and small, 

 at nearly all horizons, cherty iron carbonate, and a short distance east or 

 west ferruginous cherts and actinolitic slates. 



The question arises whether the cherty iron carbonates, the least 

 altered rocks now observable, have been derived from an earlier and 

 different rock. They are apparently unaltered, and that they were origi- 

 nally deposited in the condition now found is pro1)able enough from analo- 

 gies presented by later geological times. The cherty ironstones from the 

 Carboniferous, which occur so plentifully in our own conntr\- in Ohio and 

 Pennsylvania and so extensively in other countries, are like the.sc thinly 

 bedded carbonates, except that they are generally more argillaceous. 

 Certain Ohio ores are so remarkal)lv like .some of the iron carbonates of the 

 Penokee-Gogebic series that they can hardly be distinguished in hand 

 specimen or thin section from one another. This essential likeness is shown 

 by PI. XXVII, Figs. 1 and 2, and PI. xxix. Fig. 4. The last is an Ohio cherty 

 carbonate; the first two are Penokee carbonates. The onl\' difference 

 between the two is the unimportant one as to the nature of the inclusions. 

 The clayey character of the Carboniferous carbonates is no more than an 

 accident, and to a certain extent is also characteristic of the older carlion- 

 ates of the Northwest. It simply means that the nonfragmental sedimen- 

 tation was accompanied 1)V mechanical se<limentation to a greater extent 

 in the Carbouifen ms than in the Iron-bearing series of the Northwest. 



In Mississippi, in the Claiborne formation of tlie Tertiary,' tliere are 

 extensive beds of chert v iron carbonate, which, acconling to analyses pidi- 

 lished, are almost identical in composition with the cherty iron carlionates 

 of the Penokee series. A third analogv presented 1)\ the formations of more 



' A New Discovery of ('arbouati; Iron Ore at Euterpriae, Mississippi, by Alfred F. Brainerd. 



Trans. Am. lust. Miii. Kng., vol. xvi, 18fS8, pp. 116-149. 



