236 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
plagioclases and pseud-amygdaloidal epidote, chlorite and quartz are im- 
bedded in a groundmass consisting principally of plagioclase, with which 
are augite, commonly much changed to viridite, magnetite or titanic iron 
and epidote. This rock passes into a true amygdaloid, and is associated, 
as is also the similar rock of T. 36, R. 17 W., with diabases in all respects 
like the common types of Keweenaw Point. It is of considerable interest 
that rocks closely allied to these are found at a similarly low horizon on 
Silver Mountain, in the South Range of Michigan, and again on the Minne- 
sota coast of Lake Superior. 
In the vicinity of Saint Croix Falls, as also again on the upper Saint 
Croix, and on its tributaries, the Kettle and Snake, in Minnesota, the Ke- 
weenawan rocks are visibly overlain by the horizontal fossiliferous basal 
or Cambrian sandstones of the Mississippi Valley, the sandstones lying 
horizontally in the erosion hollows of the previously tilted Keweenawan 
beds. As this is a matter of great theoretical importance, we may quote at 
some length from Professor Chamberlin’s descriptions of the Saint Croix 
rocks.’ The other places referred to are described in subsequent paragraphs. 
At and in the vicinity of Saint Croix Falls, and southward from there to the 
neighborhood of Osceola Mills, there are numerous and very fine exposures of the Cop- 
per-bearing series and of the overlying Potsdam sandstone. * * * * Forthe greater 
part, they lie in the immediate valley of the Saint Croix river, and owe their exposure 
to the erosive action of that stream. The valley here is some 400 feet (aneroid) below 
the higher plateaus that approach the stream, within a mile or two on either hand. The 
Copper-bearing rocks appear in the slopes of the valley, at heights ranging up to 200 
feet or 300 feet. 
* * * * * * * * * 
Just below Taylor’s Falls, the river has cut a deep vertical gorge in the trappean 
rock, forming the beautiful and picturesque Upper Dalles. About two m-les below this 
a similar caton has been formed, constituting the Lower Dalles. The walls within 
these Dalles are almost absolutely vertical, and instead of showing worn faces, like 
the slopes above, present the regular rough surfaces common to fissure planes. It 
seems very probable that the original worn surface of the gorge has been riven and 
thrown down by the action of the frost and the undermining of the stream. 
* * * * * * * * * * 
In this vicinity, the relation of the Potsdam sandstone to the copper-bearing 
rocks is most satisfactorily shown. 
* * * * * * * * * 
On the N. W. 4 of See. 12, T. 33, R. 19 W., Osceola, there is an exposure of hor- 
izontally stratified sandstone in the side of a small ravine, and within a few feet is an 
1Geology of Wisconsin, Vol. III, pp. 415-421. 
