24 BULLETIN OF THE 



it is now easy to see that the ore-beds were once horizontal strata, 

 deposited iu conformity with many other stratified sediments, but they 

 are folded and broken in such a way that their true nature was fur 

 a long while misunderstood. Like the magnetic ores of the Alleghany 

 belt, they were once considei'ed eruptive ; but the progress of modern 

 science has shown that all the so-called P^ozoic iron ores are simply 

 metamorphosed strata, once deposited horizontally like the sheets of 

 iron ore now found in the unchanged Palaeozoic rocks, — such as the 

 Clinton ore and the 'black-baud' and 'clay ironstone' of the Coal 

 Measures." 



Col. Chas. Whittlesey,* in 1875, opposed the idea that the granitic 

 region was Laurentian, but regarded the rocks as eruptive. He accepted 

 the view of the Huronian age of the schists, however. 



In the Transactions of the Wisconsin Academy of Sciences, Arts, and 

 LetterSjt Mr. E. T. Sweet calls attention to the unconformability of the 

 Laurentian and Huronian with one another at Penokie Gap, Wisconsin. 

 It will be seen, however, by reference to the paper, that the absolute 

 contact was not observed. 



In 1875 Mr. Brooks again took up the qiicstion of the granites in the 

 Menomonee district, which, according to him,;}: overlie the iron-bearing 

 rocks of that region. In order to solve the difficulty he assumes that 

 they are the youngest Huronian rocks (Formation XX.), and immedi- 

 ately underlie the copper-bearing series.§ 



The only reason, so far as we can learn from his paper and the original 

 observation quoted by us ((inte, page 22), for this supposition is, that, 

 while they are lithologically identical with the " Laurentian" rocks, he 

 can dispose of them best by placing tliem as the youngest of the Huro- 

 nian rocks. Furthermore, unless he did this, the " Laurentian " would 

 be younger than the " Huronian " at tliis point, a conclusion that would 

 vitiate his former statements. He therefore deliberately violates the 

 lithological laws on which his work rests, and makes it simply a ques- 

 tion of where each rock will fit into his system the best. 



From Dr. T. Sterry H,unt's "Azoic Rocks," || we learn that from hand 

 specimens sent him by Mr. Brooks he established the presence of the 

 Montalban series, as well as the Laurentian and Huronian (/. c, p. 



* Proc. Am. Assoc. Adv. Sci., 1875, XXI Y. 60-72. 



t Vol. III., 1875-76, pp. 40-55. 



} Mich. Geol. Survey, I. 175. 



§ Am. Jour. Sci., 1876, (3,) XL 206-211. 



II Sec. Geol. Survey of Penii. E. Part I. 



