1880.] 
93 
[Wadsworth. 
conglomerate, Potsdam according to the researches of Foster 
and Whitney, and later those of Rominger, dip to the northwest 
and are covered by the old basaltic lava (copper-bearing trap) 
in a thin sheet of some two feet in thickness ; again sandstone and 
conglomerate were formed upon this lava, then another lava flow, 
then more conglomerate, and so on. This formation continues 
generally with more abundant lava and less sandstone and con- 
glomerate, across Keweenaw Point until the western sandstone 
is approached, where the lava flows diminish in number and 
the sandstone and conglomerate beds increase. In other words 
we had a gradually increasing volcanic activity which culminated 
and finally died away. This statement applies, of course, to the 
basaltic eruptions, for we as yet know but little concerning the 
rhyolitic and trachytic eruptions of Lake Superior. 
The basaltic lava was poured out on an old wave-washed shore, 
where it was often denuded and buried under beds of sandstone 
and conglomerate, sometimes miles in extent, and from a few 
inches to 198 feet, and even half a mile in thickness (Marvine). 
These sandstones and conglomerates are in part composed of 
debris of the basaltic lava, but mostly of trachytic and rhyolitic 
materials. 
These conglomerates have, in some cases, been filled with 
copper, which filling must have occurred since the conglomerates 
were formed and the succeeding lava flow had taken place, for 
the co])per is found wrapped in thin sheets around the jointed 
blocks of the hanging wall trap, and extending down into the 
conglomerate. Pebbles of trap (amygdaloid) of a few inches or 
more in diameter are frequently found in the conglomerate, 
which pebbles have had their original materials largely removed 
and replaced by copper. That this reifiacement occurred at the 
time the conglomerate was impregnated with copper is evident 
from the fact that the copper filling the trap pebbles is connected 
by stringers with that filling the conglomerate. 
In the case of the “ ash bed ” at Copper Falls, we have a loose, 
scoriaceous flow, whose interstices have been filled with littoral 
sand, and then buried under a succeeding lava flow. That the 
impregnation of this “ ash bed ” with copper occurred since the 
overlying lava was in place is shown by two facts : * 
