610 Dr. T. Sternj Hunt — The Eozoic Eochs of N. America. 



upon the various crystalline series namerl, and that all of these great 

 groups belong to Eozoic time. The Quebec group of Logan, as well 

 as what he called the Potsdam group, is this same Cambrian or 

 Transition Greywacke. The Hudson Eiver group also, as first 

 described by Vanuxem and by Mather, and later by Logan up to 

 1860 (when he changed its name to that of the Quebec group), is 

 nothing else than this same Cambrian Greywacke, with the addition, 

 in certain localities, of a portion of Taconian, and in other of 

 schistose beds containing the second fauna (Utica and Loraine shales). 

 The above explanation becomes necessary for the reason that the 

 Canadian geologists (Logan and the present writer) formerly 

 described, in accordance Avith the views of the first-named school, 

 certain crystalline schists, chiefly Huronian. as altered rocks of the 

 Hudson Eiver group, and later (from 1860 to 1867) as of the 

 Quebec group. 



The cupriferous series of the basin of Lake Superior (the distinctness 

 of which was maintained by the writer in 1873, when he called it 

 the Keweenaw group, a name which he subsequently changed to 

 Keweenian), which has a thickness probably greatly exceeding 

 20,000 feet, was also by Logan referred to the Quebec group. It 

 has, however, been shown by later observers that the fossiliferous 

 sandstones which rest in horizontal layers upon the inclined strata of 

 the Keweenian, belong to the Cambrian, and hold the fauna of the 

 Potsdam. The conglomerates of the Keweenian cupriferous series 

 contain portions alike of Laurentian, Arvonian, Huronian, and Mon- 

 talban rocks, and appear, according to the latest observations, to 

 overlie the schists which we have referred to the Taconian. The 

 sandstones and argillites of the Keweenian, which are interstratified 

 with great masses of melaphyre, are uncrystalline. It remains to be 

 determined whether the intermediate Keweenian series has greater 

 affinities with the Taconian than with the Cambrian. 



AVe have thus sought to include the whole vast system of primitive 

 and transition crystalline rocks, from the fundamental granitoid 

 gneiss upwai'd, under the names of Laurentian, Norian, Arvonian, 

 Huronian, Montalban, and Taconian. 



The Arvonian or petrosilex group intervenes between the Laurentian 

 and the Huronian, but the peculiar characters of the Norian, and its 

 localization to some few limited areas in Europe and North America, 

 make it difficult for us, as yet, to define its precise relations to the 

 Arvonian. The Norian, however, probably like the Arvonian, occupies 

 a horizon between Laurentian and Huronian. Much time may pass, 

 and many stratigraphical studies must be made, before the precise 

 relations of the Huronian and the succeeding Montalban can be 

 defined. It seems probable, in the present state of our knowledge, 

 that the Montalban series was, in many cases, deposited over areas 

 where the Huronian had never been laid down. Notwithstanding 

 the great geographical extent and the importance of these two series, 

 neither can claim that universality which probably belonged to the 

 primitive granitic substratum, a universality soon interrupted by 

 the appearance of dry land ; an event which preceded Huronian time. 



