CHAMBERLIN ON THE ROCKS OF ST. CROIX FALLS. 237 
exposure of melaphyr' of the Keweenaw series. The sandstone is quite hard and 
compact, but the stratification is undisturbed, and there is no indication of met- 
amorphism. A short distance farther north, the road passes over an exposure of 
melaphyr on which the sandstone is seen deposited in direct contact with it. 
In the south part of the village of Taylor’s Falls, near the summit of the ridge, 
is a remarkable exposure of the Potsdam in connection with the Keweenaw series. 
Its occurrence is illustrated in Fig. 7, but somewhat idealized. All that is seen is an 
outcrop of melaphyr, and, on the exposed face, 
a conglomerate formed of rounded and water- 
worn fragments of melaphyr. These frag- 
ments are of all sizes, and the cementing ma- 
terial is a ferruginous sandstone of Potsdam 
age, containing occasional Lingulepis shells. 
In the vicinity of the melaphyr the greater part 
of the conglomerate consists of its fragments. 
After inspecting the locality, it seems evident 
that the melaphyr was an exposed cliff in the 
Potsdam sea; about whose base large and 
small water-worn fragments of the melaphyr 
were collected, and, the interstices being filled Fie. aes unconformable contact be- 
with sand, solidified into the conglomerate as tween Keweenawan diabase-porphyry,and Pots- 
itnow appears. The junction of the two forma- dam sandstone at Taylor’s Falls, Minn.; after 
tions is not well exposed, as the sandstone *'°"% 
graduates into the conglomerate, and the latter is banked up against the uneven sur- 
face of the melaphyr. The unconformability, however, cannot be doubted. 
* * * * * * * * * 
On the west side of the river, about half a mile above Taylor’s Falls, when the 
water is low, the junction of the Potsdam and Keweenaw formations may be found. 
* * * * The fossiliferous blue shales of the Potsdam are horizontally deposited 
on a melaphyr, containing small specks of pyrites. The melaphyr breaks out in thin 
pieces, having the shales firmly attached. The junction is marked by a dark line 
about one-eighth of an inch thick, which seems to cement the two formations. 
* * * * * * * * * 
The foregoing facts, corroborated as they are by much other data gathered from 
the vicinity and other parts of the district, and elsewhere, make it certain that the 
Copper-bearing series was subjected to much erosion during and previous to the for- 
mation of the Potsdam sandstone, by which valleys were cut into it to the depth of at 
least 300 feet. This, doubtless, represents but a small proportion of the total erosion 
which the series had suffered in the pre-Potsdam period. 
This locality presents the most clear and unequivocal evidence that the Copper- 
bearing series is much older than the Potsdam sandstone of our state, so much older, 
indeed, that there was time for the very extensive wearing down of the former before 
the latter was deposited. 
The crystalline rocks of the Copper-bearing series of this locality, while varying 
somewhat, possess a general similarity, and are not diversified by conspicuously dis- 
tinct beds of different kinds of rock, as has been found to be usually the case in other 
1Rather a porphyritic diabase, according to the nomenclature adopted in this yolume.—R. D. I. 
