416 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
The spaces between the lines do not, of course, represent single con- 
tinuous beds, or even, in many cases, groups of beds, for in their course 
around the lake such beds must constantly thin out and be replaced by 
others. The spaces are, however, designed to cover the same general 
horizons, so far as practicable. Of course there must be many imperfections in 
such a map, under the very best of circumstances, for not only do the courses 
of the belts under the lake have to be hypothetical, but, from the general 
similarity of the beds of the Lower Division at very different horizons, there 
must always be more or less doubt as to the correctness of the connecting 
lines, even on the land. Then, again, the map is very irregular as to accu- 
racy in those places where the courses and inclinations of the beds can be 
marked out. On Keweenaw Point, for instance, the detailed measurements 
of Pumpelly and Marvine make it possible to locate the courses of the 
2,500-feet spaces with far greater minuteness of detail than it is possible to 
show on such a map as this. From this downwards there is every degree 
of accuracy to cases where the lines are purely hypothetical. 
Notwithstanding all these defects, the general correctness of the struct- 
ure of the great synclinal indicated by the red lines appears to me to be 
beyond question. One objection that I anticipate to this map is that it is 
an attempt to apply the methods used in studying sedimentary beds to a 
series largely formed of eruptive ones; to which I have to answer beforehand 
that this series is just as much made up of layers as any sedimentary one, 
and that in a sedimentary series beds thicken and thin and disappear just 
as here. 
The map and sections do not show the nature of the bottom of the 
trough. I believe this bottom to be made up of Huronian slates below, 
resting upon the older gneiss, and of Keweenawan strata above, but both 
greatly thinned, since the eruptive rocks, which constitute so large a part 
of these groups around the edge of the basin, appear to me to have reached 
the surface there. 
The simplicity of the synclinal has been further complicated by 
faulting. The fault to the south of the Keweenaw Point Range, it seems 
probable, may have been connected with a sudden change in the dip of the 
strata from a flat to a steep lakeward inclination. It seems a plausible 
