96 PROCEEDINGS OF THE WASHINGTON MEETING 



this figure. (3) Therefore I have changed the hade of the fault from that of 

 an overthrust to the steep hade of a block-fault. There is some direct evi- 

 dence of this. It is certainly not as flat as a later overthrust fault (Old 

 Colony and Torch Lake and Oneco sections) . A nearly vertical hade would 

 be also a more natural channel for volcanic activities. Many overthrust faults 

 are devoid of them. 



Primitive Erosion 



We agree, as shown in figures 3 and 4, that this primitive fault suffered 

 erosion, and that the Eastern or Jacobsville sandstone may have lapped on 

 both sides of it. But the study of land formations, which has much advanced 

 in the last 20 years, would lead one to infer that the valley at d was being 

 filled as it was formed, and that there is no such sharp gap in dip or strike 

 or time of deposition between the Upper Keweenawan and the Cambrian sand- 

 stone, as is fairly inferable from figure 3. Tn fact, my own figure 4 does not 

 do justice to my conception of the continuity of sedimentation that takes 



Figure 4. — A Stage corresponding to Figure 3 

 Showing the section after the deposition of the four Paleozoic overlaps 



place in a block-fault valley or graben. The Keweenawan is for me largely the 

 land facies (the Old Red Sandstone facies) of the Middle and Early Cambrian 

 (Waucobic). We know now definitely through Thwaites and the Wisconsin 

 Survey that the Upper Keweenawan is indistinguishable in lithology, dip, or 

 strike from the Cambrian sandstones, for the "Western" Apostle Islands sand- 

 stones that have been hitherto called Cambrian by every one he classes as 

 Upper Keweenawan, and the line between them and the Mississippi Valley 

 fossiliferous Upper Cambrian sandstone is drawn in a region where there are 

 no outcrops. However, Limestone Mountain at .;' has fortunately been left us 

 (figure 8) to show that the "Eastern" sandstone is really directly beneath the 

 Trenton. The Jacobsville or Eastern sandstone shows both in drill cores (as 

 at the Oneco mine) and exposures fragments of the Keweenawan amygda- 

 loids, that indicate clearly that at the deposition there was yet a little of the 

 uplifted fault-block exposed to attack. The association of zeolites with copper 

 shows the cooperation of thermal waters, and I therefore believe that much, 

 if not all, of the copper deposition took place during this period of early 

 erosion. 



