998 The Crystalline Rocks of the Northwest [ October, | 
with gabbro group. In the term Montalban proposed for these 
groups by Dr. Hunt, the two are united and the constant distinct 
ness which they seem to maintain is not recognized. The granite 
and gabbro group has affinities with the onlying Cupriferous rocks, 
and perhaps, as Irving has suggested, should be considered the 
base of that series which Brooks has named “ Kewenawian,” 
whereas the mica schist group has affinities with the underlying 
groups, and has, without exception, been assigned to the same 
system and age as those underlying groups, The granite and 
gabbro group has likewise been designated differently. The gab- 
bro, being an igneous rock, varies much in its prevalence and ia 
its apparent relation to the granite. Its greatest development 
produces in Minnesota a range of low hills which extend north. 
eastward from Duluth. Under similar circumstances, this gon 
has received the name Norian, though at first called Labradonan, 
and thought to be a part of the Laurentian! The granite and 
gneiss, also, associated with the gabbro, have received, under one 
of their modified conditions, the special designation Arvontan, on 
the supposition that these rocks where they so appear, are not 
modified conditions of granite and gneiss, but represent indè- 
pendent strata that lie near the bottom of the “ Huronian” equal 
in rank to any of the other groups. I think I have shown ele 
where? that the Arvonian rocks are interstratified with the Cupr- 
ferous, and also that they are modified sediments of the Cuprifer: 
ous. Instead of being near the bottom of the “ Huronian f 
the Northwest, they overlie all the groups that have been assigned 
to the Huronian by Irving, and constitute a part of the great sè- 
ries of “ younger gneisses ” which by Brooks has been ranked 
the “youngest Huronian.” he 
The interesting variety of nomenclature as brought out byt 
foregoing remarks, can be seen by a glance at the accompanying 
tabular arrangement, where the various parallelisms and the co 
flicting nomenclature are placed in adjoining columns. d 
It is evident from this table that at present it is a hazardo p 
and perhaps an impossible, undertaking to assign the grok 
the crystalline rocks of the Northwest to any of the terranes i 
have been named further east, without violating somebody 7 
tem of nomenclature. Some of the ground has been | 
* It was described by Emmons under the term “ H ypersthene rock.” 
7 A. A. A. S. Cincinnati meeting; Minnesota Survey Rep. for 1880, P. BOGE 
1881, p. 110. a 
