106 Copper Minhuj in Miimesohi — IlaJJ. 



This area is not large; it is the south westward extension of the 

 southern portion of the Lake Superior synclinal, that which pre- 

 sents its most typical exposures on Keweenaw Point. For many 

 miles occasional exposures of these rocks may be seen along the 

 St. Croix river, which forms the boundary line for a hundred miles 

 or so, between the two states of Minnesota and Wisconsin. The 

 exposures along the Snake and Kettle rivers, two Minnesota 

 branches of the St. Croix, are somewhat scattered, and at the same 

 time are fewer than those along the St. Croix. So far as is known 

 to the writer these Keweenawan rocks in Minnesota die out com- 

 pletely on the east side of the St. Paul and Duluth railroad; not a 

 single exposure is yet reported from the west side of that road, un- 

 less the numerous dikes of the granite area of Central Minnesota, 

 in Stearns and neighboring counties, be regarded as of Keweenawan 

 age. That these dikes are of this age is very possible, nay proba- 

 ble. 



Other views than the one above given have been held by geol- 

 ogists, with reference to the age of these copper-bearing rocks; 

 they have, in short, been relegated to positions from ths Jurassic 

 down to the Huronian. The opinion of Irving, above stated, is 

 both the latest one and the one best and most completely support- 

 ed by field evidence: for those two reason-^ the writer is willing to 

 accept it until some competent person has reviewed the ground 

 and found grave faults in Irving's observations, arguments and 

 conclusions. 



The rocks which are included within this Keweenawan group 

 are both sedimentary and eruptive. The sedimentary in Minneso- 

 ta are red sandstones, such as occur on the north shore of Lake Su- 

 perior, among other places at Good Harbor bay, in a bed upward 

 of 200 or more feet in thickness, dipping southwesterly at an angle 

 of 8 to 10 degrees, and conglomerates as those a mile or two below 

 (east of) the mouth of Poplar river. Above, and below these sand- 

 stone and conglomerate layers are massive flows of diabasic rocks, 

 which in places are quite amygdaloidal with calcite. laumontite, 

 thom^onite, lintonite, stilbite, etc. Further, sedimentary rocks 

 appear in the St. Croix valley in the form of conglomerates as- 

 sociated with ash bed "trap" and amygdaloidal rocks of a diabasic 

 type. Several localities can be mentioned. 



At Taylors Falls melaphyre stands up thro' the Cambrian sand- 

 stones and fossiliferous shales. In places, as along the bank of 



