STRUCTURAL GEOLOGY. 63 



Effect on the Aiiiinikie the red rock.] 



the source and the cause of the red rock may have been deep-seated, and may 

 have involved other formations than the Animikie. 



If this general conception of the origin and date of the red rock be admitted, 

 the surface distribution which it shows in Minnesota, and, so far as known, all the 

 special features of contact, flowage, intrusion and alternation with the gabbro and 

 the cotemporary diabases, are easily apprehended. 



In the main, the line of strike of the Animikie from Pigeon point to Duluth is 

 represented by the red rock belt. It is certain that the undisturbed line of strike of 

 the Animikie leaves the gabbro area and runs more westward, constituting the 

 Mesabi iron range. It must therefore be understood that the strike of the red rock 

 belt further south, so as to reach Duluth, marks a line of upheaval through the body 

 of the Animikie area, really constituting another strike-line, or fracture line, 

 dependent, not on the earlier existence of a shore line, but on the direction of a 

 zone of dynamic fusion of later date. This red rock belt comprises the water divide. 

 From it streams flow north and south toward the north across the main gabbro 

 area and on to the Archean, toward the south across the later diabases, etc., of the 

 Keweenawan, into the area of the sandstones of the lake Superior valley. At 

 Duluth the main gabbro area unites with and blends into the great Beaver Bay 

 diabase, lying sometimes upon the red rock and sometimes on metamorphic rocks 

 referred to the Animikie. It is probable that this relation occurs at other places 

 toward the northeast, especially in St. Louis county, but they have not been observed, 

 and therefore the red rock is considered as a continuous belt, while in many places 

 it is known that the gabbro is separated from the Beaver Bay diabase by a wide 

 tract of red rock. 



The greater endurance of the red rock may be in part the cause of this greater 

 elevation, and it may be in part due to a greater original elevation of the Animikie. 

 The latter is indicated by the fact that toward the east this greater elevation blends 

 in with the area of the Animikie, and its sills in Cook county, as the gabbro area 

 becomes smaller and narrower. From the point at which the present water divide 

 crosses the international boundary, between North and South lakes, the upper 

 waters of Pigeon river, including the lakes of the international boundary, are turned 

 eastward along the north side of this belt of greater elevation to the vicinity of 

 South Fowl lake, below which point the Pigeon river begins its tumultuous descent 

 to lake Superior, crossing the belt of elevation at its lowest (eroded) passage, and 

 often running in a sharp preglacial gorge-like valley several hundred feet below the 

 adjoining summits. 



The Beaver Bay diabase. This term is not used in exactly the sense employed 

 by Irving, although its rocks are embraced, in general, in Irving's Beaver Bay 



