Geology of the Lake Superior Land District. 21 
We would merely observe that in this region the iron masses are 
invarjably found in this association—never oceurring in the 
granite. 
These alterations of trappean and schistose rocks continue, to 
near the southern boundary of township 47, and are characterized 
in many places by the ores above describe d. 
Section from Lake Superior to Lake Michigan.—The coast 
near the head of Keweenaw Bay (L’Anse) aflords an admirable 
section of the slates and the overlying sandstone 
The following is the descending order of succession : 
1 and 2. Fissile sandstone—the equivalent of the Potsdam— 
dipping slightly to the west-northwest, of a reddish color, and 
coarse-grained, passing into a conglomerate comiposed of peb- 
bles of t milk-white quartz, and occasionally trappean pebbles— 
13 feet, resting ee on the azoic rocks, consisting of 
3. Chlorite slate and novaculite, or silicious slate variously 
colored, and much obnibiededs places folded o 
4. A dark hornblende and feldspar rock aeannty: ne Gate in 
its origin. 
Formations 3 and 4 are traversed by veins of quartz which in 
no case penetrate the overlying sandstone. The slates are also 
occasionally intersected by dykes of tra 
This section is exceedingly instructive, inasmuch as it enables 
Us to draw a line of demarcation between two formations ond 
ferent in age and external characters. While the newer form 
places, invaded by igneous rocks. Their structure has been 
changed from granular to sub-crystalline, and the whole mass is 
toon by numerous planes of lamination. 
* * 
* 
Thickness of sis System. Foldings of the Strata.—All 
attempts to estimate the thickness of the various schistose, cal- 
careous and quartzose beds, must prove merely approximative. 
ey occupy a. belt which in its widest expansion reaches not 
less than eighty miles in width, and wherever exposed, have an 
inclination approaching verticality. If we were to deduct the 
Spaces occupied by the purely igneous rocks, and then measure 
across the basset edges of those supposed to be sedimentary, the 
result would give us an incredible thickness—a thickness far 
Surpassing that of the whole series of rocks heretofore observed, 
from the base of the Silurian to the crowning member of the 
tertiary. It is highly probable that the beds are arranged in a 
pita: of flexures, and that the observer in passing over the out- 
cropping edges, oe numerous repetitions of the same beds ; 
they have, however, be so repeatedly shattered by uak 
a 
