250 DR. THOMAS STERRY HUNT ON THE 
$ 88. In comparing these rocks in Maine, New Brunswick, Ontario and northern New 
York with the similar rocks in the Appalachian valley, and elsewhere southwards, it 
should be remembered that in these latter regions the strata in many cases present at their 
outcrops, soft materials, the results of sub-aérial decay ; whereas only their harder under- 
lying portions are seen in the eroded regions further northward. These varying conditions 
of outcrops of similar crystalline rocks in different geographical areas have elsewhere been 
discussed by the writer. * 
$ 89. That the fauna of the Lower Cambrian, as seen in the Menevian beds of our 
eastern coast, or in the so-called Potsdam, which forms the base of the Cambrian in 
Minnesota, and in Wisconsin, marks the dawn of organic life, will now scarcely be main- 
tained, even by those who question the organic nature of Eozoon. Recent observations 
show the existence, beneath this horizon, to the westward, elsewhere, of fossiliferous strata 
occupying the place which we have assigned to the Taconic. There is found, along the 
northwest shore of Lake Superior, a series of quartzites impure limestones, and slates, 
which the writer has called the Animikie group. These had been, by Logan, assigned to 
a position at the base of the great Keweenian or copper-bearing series of the region. The 
writer, in 1873 and 1878, attempted to show that the conglomerates, sandstones and lime- 
stones of the so-called Nipigon group, overlying the Animikie, are not the Keweenian, but 
newer rocks ; which, so far as the evidence afforded by the sections examined by Logan 
and himself, at and near Thunder Bay, goes, might either be of Potsdam, as maintained by 
some, or, as held by others, from lithological considerations, of a more recent age. 
§ 90. The Animikie group, the distinctness of which from the overlying Nipigon was 
then maintained, has since been traced westward by N. H. Winchell, and shown to underlie 
unconformably the horizontal sandstones of the St. Louis River, regarded as belonging to 
the Potsdam or Lower Cambrian of the region. In this case, the Aminikie quartzites and 
slates, which rest unconformably upon the Huronian, would occupy the horizon of the 
Lower Taconic. The writer has recently found in the argillites of this series, near 
Thompson, in Minnesota (in the vicinity of which they afford roofing-slates), numerous 
calcareous concretions, one of which has yielded to Dr. J. W. Dawson the remains of a 
keratose sponge. t+ The granular quartzites or sandstones of this series are often mingled 
with magnetite ; and constitute in some cases, as described by Dr. Bell, an iron-ore. 
In this connection it should be mentioned that the quartzites and argillites of the 
Menominee River, in northern Michigan were, in 1846, referred by Houghton and Emmons 
to the Taconic system.{ There is reason to believe that these rocks of the Menominee 
region which, as described by Brooks, and Pumpelly, include great deposits of iron-ores 
and marbles, and apparently differ much from the Huronian in character, are, as supposed 
by Irving, identical with the Animikie rocks. 
§ 91. Major Powell, the director of the United States geological survey, in 1882, 
re-examined in the Grand Canon of the Colorado, a series of rocks, previously designated 
by him the Grand Canon group, and described as consisting of over 10,000 feet of uncrys- 
talline rocks—sandstones, limestones and slates,—resting unconformably upon older crys- 


* The Decay of Rocks; Amer. Jour. Science, 1883, xxvi, pp. 190-213. 
+ This section has been added since the presentation of this paper, the specimens haying been collected in 
August, 1883. Their examination is not yet completed.—T. 8. H. 
{Emmons; Agriculture of New York, p. 10. 

