The Eozoic Rocks of North America. 87 



(Logan and the present writer) formerly described, in accordance 

 with 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 dis- 

 tinctness 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 Montalban 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 mela- 

 phyre, are uncrystalline. It remains to be determined whether 

 the intermediate Keweenian series has greater affinities with the 

 Taconian than with the Cambrian. 



We have thus sought to include provisionally the whole vast 

 system of Primitive and Transition crystalline rocks, from the 

 fundamental granitoid gneiss upward, under the names of Lau- 

 rentian, Norian, Arvonian, Huronian, Montalban and Taconian, 

 The Arvonian or petrosilex group intervenes between the Lauren- 

 tian 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 Huro- 

 nian. Much time may pass, and many stratigraphical studies must 

 be made before the precise relations of the Huronian and the suc- 

 ceeding 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 



