CHAPTER III. 



By R, D. Irving and C. R. Van Hise. 



THE CHERTY LIMESTONE. 



Eelation of thi' limestonn ami cluTt. (TiMigraiihiral ilistribution. Possible former greater continuity. 

 Thiekness. Petrographical character of the limestone, ri'troffrapliical character of the chert. 

 Change to the overlying Quartz-slate. Tabulation of petrographieal observations. Prominent 

 exposures. Origin of the limestone ami chert. Summary. 



Relation, of flic limestone and chert. — In tliL- tliii-d vohiine of the Geol-, 

 ogy of Wisconsin (pp. 104, 106-108), and on Pis. xxiii t() xxvi of tlie atlas 

 accompanying- that voliiine, the lowest inenil)erof the Iron-bearing series of 

 the Penokee district is represented as being-atreinolitic crystalline limestone, 

 with an average tliickness of 90 feet, above which is said to sncceed a 

 peculiar wliite quartzite, with an average thickness of 40 feet. These 

 two layers, though occasionally wanting altog-ether, as indicated on the 

 maps referred to, are represented as ha\ing a considerable longitudinal 

 extent and as being separate from one another: that is to say, all of the 

 white quarts is taken as lying above all of the limestone.' Phat this 

 is the case was concluded particularly from the exposures seen at Penokee 

 gap, where such a successio'ii is very manifesth' the true one, as indicated 

 on PI. XXVI of the Wisconsin atlas. Our later investigations, and more par- 

 ticularly our experience on the Michigan side of the Montreal river, have 

 shown us, however, that in fact these two rocks form portions of one and 

 the same layer; that is to say, that the white quartz, or chert, as we now 

 think it shoidd more properly be called, occurs as a rule interstratiiied and 

 thinly interlaminated with the limestone. Taken together, these two ma- 



' However, a doubt was expressed (Geol. of Wis.*, vol. m, 1>*K0, p. 110) as to whether the 

 limestone exposures iu the westi-rii p.irt of the Peuoliec district might not represent both limestone 

 and chert as seen at Peuokee gap, 



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