726 WALTER H. BUCHER 



The map accompanying Thwaites's paper shows that the jointed 

 area lies in the continuation of the northeastern end of the great 

 Douglas thrust fault. If this fault had any horizontal component 

 in the northeasterly direction, it could have supplied the com- 

 pressive stress responsible for the position of the joints. 



Unfortunately, the fault contact of the Middle Keweenawan 

 traps with the underlying much younger Orienta sandstone was 

 found exposed only at four localities. At three of these, the 

 exposures were not even found sufficient to measure the hade 

 of the thrust plane. ^ 



In the vicinity of the falls of the Amnicon River, however, 

 the fault-plane proper was found exposed at two separate localities, 

 about 500 feet apart. Here, at both points, two systems of grooves 

 were observed on slickensided surfaces in the immediate vicinity 

 of the fault, on surfaces of conglomeratic beds of sandstone which 

 represent most probably shreds of lower beds dragged up along 

 the thrust-plane.^ One of the sets of grooves "is parallel to the 

 dip, the other is inclined at an angle of about 30° in a NE-SW 

 direction. "3 



Although the grooves are not part of the fault-planes proper, 

 but occur on what seem to be irregular fragments of sandstone 

 wedged in front of the fault, their constancy on seemingly different 

 planes at points 500 feet apart can hardly be looked on as due to 

 purely local movements. They are more likely the direct result 

 of the last movements along the major thrust-plane and essentially 

 parallel to them. 



If the joints in the vicinity of the Apostle Islands really owe 

 their origin to the action of this pressure directed upward at an 

 angle of about 30° toward the northeast, we should find evidence 

 of it in the position of the joint planes themselves. According to 

 the table on page 96 of Thwaites's paper the joints have the 

 tendency to be vertical. From the text we learn, however, in 

 addition that "many of the E-W joints are inclined, usually at 

 a steep angle to the north. "^ 



' F. T. Thwaites, op. cit., pp. 66, 76, 80, 81. ' Ibid., p. 83. 



3 Ibid., p. 78. " Ibid., p. 94. 



