364 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
that there can be no doubt as to the source of the pebbles. The more dis- 
tinctly quartzose character of the Eastern Sandstone, as compared with the 
detrital beds of the Keweenaw Series, finds its explanation, I think, in the 
derivation of the larger part of its material from the granites and schists of 
the region south and east from Keweenaw Bay. Although Mr. Wadsworth 
feels obliged to explain away its silicious character by a supposed process 
of leaching by hot water, the lithological character of the Eastern Sand- 
stone, but for its carrying pebbles of Keweenawan eruptives, would be 
more in favor of his view than against it. If the Eastern Sandstone ante- 
dates all of the Keweenaw Series it should present a strong lithological 
contrast with the detrital rocks of that series, having been derived from a 
wholly different source. 
Moreover, there are general-considerations which would make this view 
impossible of acceptance, even were the occurrences at the contact not so 
conclusively against it as they are. If the whole mass of the Keweenaw 
Point succession overlies the Eastern Sandstone, what has become of this 
seven miles of rock thickness to the eastward? It will not do to say that we 
are dealing here with eruptive rocks which thicken and thin suddenly, and 
cannot therefore be reasoned about in the same manner in which we would 
deal with beds of sediment. No eruptive agencies ever pile up seven miles 
of rock with a vertical wall of that height extending over a hundred miles 
in length. Besides, in this case the eruptive beds or flows are structurally 
just like beds of sediment, which thicken and thin also. Yet more, fully 
two miles of the thickness is of sediment. Nothing but an immense erosion 
on Mr. Wadsworth’s view of the inferior position of the Eastern Sandstone 
can explain the disappearance of so great a thickness of strata. But an ero- 
sion which has stopped suddenly on so sharp a line, parallel to the general 
trend of the layers, and yet has left nowhere behind this line a trace of the 
former extent to the south and east of this immense thickness of resistant 
beds, while leaving undenuded over a wide area an inconsiderable thickness 
of an underlying fragile sandstone, is incredible. Again, east of Marquette 
the Eastern Sandstone appears to pass upwards insensibly into the beds of 
the Calciferous. Yet on Keweenaw Point, only a few miles away, this the- 
