MICHIGAN AND WISCONSIN. 497 



Hiironian. There seems not the slightest reason to regard this as any- 

 thing different from the so-called Laurentian granite, and this strengtii- 

 ens the probability of the view that the so-called Laurentian granite is 

 really eruptive in the so-called Huronian. {Ante, pp. 22, 70, 71.) 



In 1880, Professor Irving gives as the reasons for assigning the rocks 

 which are placed in the Laurentian in "Wisconsin to that system, their 



" close lithological similarity — the only marked diflference being the absence 

 of crystalline limestones in the Wisconsin area — of similar structural rehitions 

 to the Huronian, Keweenawan, and Lower Silurian systems, and of probable 

 direct continuity with the Canada Laurentian through the upper peninsula of 

 Michigan and underneath the waters of Lake Superior," (Trans. Am. Inst. 

 Min. Eng., ISSO, YIII., pp. 480, 481.) 



Of the Huronian in the same article it is stated (p. 483) : — 



" The rocks of this series have been called Huronian by Brooks, and, in the 

 WTiter's judgment, correctly so, on account of their similarity to the Canada 

 Huronian, Avith which they not improbably have a direct connection under- 

 neath the Silurian of the eastern part of the peninsula, but more especially 

 because they evidenth' occupy the same geological interval as the typical 

 Canadian series, exhibiting the same non-conformity with an underlying 

 gneissic and granitic system." 



It appeal's, then, that the only evidence that the Wisconsin geologists 

 have that the Laurentian and Huronian are what they purport to be is 

 lithological ; and they have advanced no sound argument showing that 

 they form distinct ages in the Azoic System. The relation of the two 

 supposed series is not that which is seen when the Palseozoic comes in 

 contact with the Azoic, or what it would be naturally were the Huro- 

 nian laid down on the pre-existing Laurentian. The contacts — when 

 these contacts have been figured — appear rather to be those made by 

 eruptive rocks with prior existing ones. The geologists before men- 

 tioned have assumed, not proved, the sedimentary metamorphic origin 

 of all the rocks in question, and on the correctness of that assumption 

 depends their argument. They have failed to observe the phenomena 

 of the contact, when seen, beyond the mere f\\ct of a different dip to 

 the foliation observed. In fact, they have failed to prove any of the 

 points essential to establishing their conclusions. 



Later, Irving advanced, as if original with himself, the view that the 

 so-called greenstones or diorites of the Marquette district were eruptive, 

 and that the hornblende was secondary after augite. This too when 

 the portion of this Bulletin relating to the Iron and Copper districts of 

 Lake Superior {ante, pp. 1-157) had been sent him, in 1880, at his own 



VOL. ATI. — NO. II. 32 



