GEOLOGICAL EXPLORATIONS AND LITEKATUEE— 1885. 105 



in believiny tliat tliL^ "dioritic g-roup" of this uuthor is the basement upon 

 whieli the rest of tlie series was spread. The lueniljers of tliis "<i'ronp," if 

 sediineutarv, are in a highly metamorphosed condition. 



Where these greenish schists coiue into contact with the bounding granite the 

 latter penetrates them iu the most intricate manner, so that we can not resist the 

 conclusion that it is the more recently formed rock. From this unmistakable relation, 

 regarding his Dioritic Group as the lowest member of the slaty or iron bearing .series, 

 Dr. liominger naturally passes to the conclusion that the granites are, in large meas- 

 ure, subsequent to his entire series. * * * To me, however, it seems plain that in 

 the greenstone-schists at the base of the Marqnette iron-bearing series we have the 

 equivalents of those * * * south of the Peaokee-Gogebic iron-bearing series, 

 like which tlicy form, as I conceive, part, not of the higher, but of the lower forma- 

 tion. * * * The slate series above the greenish schists, in the main composed 

 of I'elatively little altered rocks, was originally built up npon a basement composed of 

 granite, gneiss, and these greenish schists themselves, and subsequently was i)ushed 

 into trough-like forms by lateral pi'essnre. (Pp. 245-246.) 



The proofs given in support of this view are the same as those advanced 

 in the case of the Penokee district. 



The penetratiou of the greenish schists by the granites where the two come into 

 contact, as contrasted with the entire absence of any such relation where the bounding 

 granite forms contacts, as it doesi at a number of places, with the slates and quartzites 

 above the greenish schist group; the occurrence in the lowrr series of only highly 

 altered sediments, gneiss and granite, while the higher rocks are relatively little 

 altered; the occurrence in the higher series of fragments from the lower, "recom- 

 posed" rocks, occurring at points where the quartzites of the upper series come into 

 contact with tlie gneisses of the lower — all of these arguments hold here as well as in 

 the Penokee region. Here, then, again it seems to me plain that we have to deal with 

 a lower or greenish-schint, gneiss-granite member, and a higher, uncontormably overlying, 

 slaty, iron-bearing member. (P. 246.) 



Irving thus argues that there are two portions of the Azoic or Arcliean 

 series iu the Marquette district, and that the two portions are separated bv 

 an unconformity. To these two parts lie gives the names Laurentian and 

 Huronian, following Logan's example for the region north of Lake Huron. 

 The names are not new, nor is the idea new that the pre-Keweenawaii 

 rocks of the Marquette district may be divided into two series. But we 

 have here for the first time sufficient reasons given for their separation into 



