THICKNESS OF THE LOWER DIVISION. 157 
bays, and reach a thickness, according to Bell,’ of about 1,300 feet. These 
are the sandstones to which Hunt has proposed to restrict the term Nipi- 
gon Group,’ and which he considers as newer than the Keweenawan, but L 
satisfied myself on the ground that Logan was correct in placing them 
directly beneath the whole mass of Keweenawan diabases and amygdaloids 
of the east side of Black Bay, and that they rest with slight discordance 
upon the nearly horizontal Thunder Bay slates. It is these slates that Hunt 
has called the Animikie Group,® he regarding them also as newer than the 
Keweenawan proper. I look upon them, however, as beyond question the 
equivalents of the iron-bearing Huronian of the South Shore. The rela- 
tions of these several groups are considered more especially on a subse- 
quent page, and are merely mentioned here, because I know of no other 
instance, in the entire extent of the formation, of the existence of such 
a thickness of detrital rocks at so low a horizon. Only forty miles south- 
westward from their occurrence on the east side of Thunder Bay, at Grand 
Portage Bay, the intervening space being water-covered, the Keweenawan 
diabases rest directly upon the slates of the Animikie Group, without any 
intervening sandstone. 
The thickness of the Lower Division is always enormous, and may be 
placed, in round numbers, at from 25,000 to 30,000 feet. From this figure 
there are of course some great variations, and yet, considering the way in 
which most of the series has been built up, the variations must be regarded 
as surprisingly small. With the exception of the unusually great and sud- 
den thinning in the Bad River region of Wisconsin, it does not appear 
probable that throughout all of its geographical extent the thickness of the 
Lower Division ever sinks much below 25,000 feet. 
In the eastern part of Keweenaw Point the maximum thickness of the 
Lower Division at surface is some 25,000 feet. This measurement, however, 
does not go to the base of the series, but only to the junction with a newer 
sandstone, which overlies the continuation downwards of the Keweenaw 
Series, or rather both a repetition of more or less of the thickness included 
1 Report of the Geol. Survey of Canada, for 186769, p. 319. 
2 Second Geological Survey of Pennsylvania ‘“‘Azoic Rocks” ‘‘E,” pp. 240, 241. 
3Loe, cit., p. 240. ° 
