Correspondence. 213 
nary, conspicuously fragmental, and fine-grained-schistose; B. Agglomer¬ 
ate or conglomeratic rocks. These are carefully described, but there is 
no occasion here for reproducing the descriptions. The quartzites possess 
few peculiarities. The breccias are various, but predominantly quartzose, 
and generally with a matrix containing quartz ayd feldspar. Some of these 
are suspected to be igneous in origin, and on© seems to possess the charac¬ 
ters of a volcanic ash. Among all the rocks described, I find no descrip¬ 
tion answering the characters of the great “Plummer argillites” or “slate 
conglomerates” of Logan, and I am uncertain whether this member of the 
Huronian passed under professor Bonney’s observation. 
The author naturally conceived the view common to the Canadian geol¬ 
ogist (but erroneous as I think) that the Huronian embraces the entire 
complex of beds to the “Laurentian gneiss.” Evidently, in passing off the 
gneisses, he arrived at the usual belt of crystalline schists—the “Vermilion 
group” of the Minnesota Survey. In proceeding from these, a transition 
was observed toward less crystalline rocks, and in the midst of these was. 
the condition which I have designated “nascent mica schist” in which the 
mica folia are exceedingly minute. Still beyond, the mica is still less con¬ 
spicuous, the quartz and feldspar more soiled and much mingled with 
particles of “dust.” This is my graywackenitic rock—though not well 
constituted graywaeke. It escapes clearly from the group of crystalline 
schists. In northeastern Minnesota, this is succeeded by sundry conditions 
of earthy schists—argillitic, sericitic, chloritic, jaspilitic and hsematitim 
All these are wanting in the vicinity of the Thessalon, Missasagui and 
Blind rivers, as also the greywackenftic and crystalline-schistic rocks. It 
may be they are wanting in the vicinity of Sudbury. In the valley of the 
Thessalon, some twenty miles from its mouth, the dark, siliceous argillites 
appear, which lie near the bottom of the proper Huronian series. I take 
the liberty to say “proper” Huronian, because I find these recognized Hu¬ 
ronian rocks, northwest of lake Superior, succeeded downwards by a break 
which makes them necessarily the lower limit of a system.* I do not re¬ 
gard therefore, as Huronian, the series of rocks succeeding the Plummer 
argillites (Animike slates) downward, though the Canadian geologists may 
so regard them. I entertain a suspicion that most of the rocks investigated 
by professor Bonney belong to the lower series. Even among these, as I 
have just stated, are two groups, before we reach the “Laurentian gneiss.” 
Of these two, the upper sub-Huronian group, is embraced under Dr. Law¬ 
son’s term “Kewatin,” but is not co-extensive with it. The lower is the 
“Vermilion group” of the Minnesota Survey, to which Dr. Lawson applied 
also the designation “Couchiching group.” 
Now professor Bonney notes the evidence of rocks of widely different 
age within the compass of the series pointed out by the Canadian geologists 
as “Huronian.” His conclusions are in part as follows: 
“Among the rocks in this region at present referred to the Huronian, 
two groups may be distinguished, depending on the degree of alteration 
observed.” “This distinction must indicate either (a) that selective meta- 
* This is a stratigraphic unconfoimity which I have described in American Geologist 
Jan., 1888, and more in detail in the XVItli Annual Report Minnesota Geological {Sur¬ 
vey , pp. 256-259, 264, 323. 
