GEOLOGICAL EXPLORATIONS AND LITEEATUEE. 81 



Turning now to the columns showing the equivalents of these several 

 groups in ^lichigan, Wisconsin, and Minnesota, we note in the first place 

 that Brobks's horizon No. xx, which is made esjjecially tor the granite south 

 of the Menominee river in northern Wisconsin, appears to us, on the 

 contrary, to belong to the lowest one of Winchell's groups; in other words, 

 to be nothing more or less than a })art of the great basement series upon 

 which the great thickness of stratiform rocks wat; originally spread; that 

 numbers 1 and l(( of the Black river succession, Wisconsin, belong simi- 

 larly to this low horizon ; that while we would put the Duluth gabbro and 

 the Brule mountain red rocks at the horizon given (noting, however, the 

 total dissimilarity of these red rocks to the granitic rocks just referred to), 

 the reddish rocks of Beaver bay, on the other hand, ai'e at a very uuicli 

 higher horizon, well up in the Keweenaw series; that numbers xx and xxir 

 of the Penokee series are, as they are followed eastward from tiie gap, 

 found to consist mainly of Ijut little altered fragmental rocks, the mica- 

 schist being in fact an alteration ])hase of a merely fragmental graywacke; 

 and that the Animikie series, taken as a whole, appears to us to have a 

 thickness several times as great as indicated l)v Prof Winchell, and to cor- 

 respond to the wliole tliicknessof those rocks, which on tlie south shore of 

 lake Superior intervene between the gabbro and tlie Lower Huronian 

 rather than to be ecpiivalent to so small a portion of them. 



Irving (E. D.). Divisibility <>(' the Arcliean in tlie JSorthwest. Extract tiom 

 Address as retiriug President of tlie VViscoiisiu Academy of Sciences. Delivered 

 December 30, 1884. Published in the Am. Jonr. Sci., Md scr., vol. xxix, ISS."), pp. 

 237-249. 



This paper presents in brief certain arg'uments for a belief in the 

 divisibility of all of those rocks which in the Northwest lie beneath the 

 base of the Keweenaw series into two whollv distinct groups, separated 

 from one another by a great unconformit}', tlie uppermost of the two being 

 in its turn separate<l by a great discordance from the Keweenaw or co|iper- 

 bearing series. These unconformities indicate the intervention lietweenthe 

 several groups of periods dm-ing which prolonge(l denudation nf land sur- 

 face was being carried on. These two groups are, lieginming lielow: (1) 

 The great basement complex of gneiss, granite, and various schistose rocks; 

 MOH XIX (> 



