STRUCTURAL GEOLOGY. 57 



Extension of the gabbro toward the east.] 



slope of the Animikie dip but preserved by the greater endurance of the Logan sills, 

 the latter still occupying by far the greater part of the superficies. It is in this por- 

 tion of the northern border of the gabbro that the Animikie is very rigid and quartzose 

 although slaty and easily removable by reason of its horizontal fissility and its 

 frequent jointage. Into such a rock, undergoing upheaval and fracture, an igneous 

 rock would farthest penetrate along planes of its easy parting, which, in this case. 

 was along the sedimentary planes of slatiness. Some of the sills thus formed can 

 be seen to be continuous occasionally for several miles, but generally they sink away 

 and are replaced by others a little further north or south, after running less than two 

 miles. As soon, however, as this quartzose and slaty member of the Animikie is 

 replaced, at the point of intrusion of the gabbro, by the fragile Grand Portage gray- 

 wacke, which breaks as easily in one direction as another, the form of the molten 

 intrusion is changed into great vertical dikes,, and these prevail in the Indian Reser- 

 vation at Grand Portage. Such dikes form the mountainous features of the Grand 

 Portage region. They are sometimes 100 or 200 feet wide, and they stand above the 

 surrounding valleys 400 to 800 feet high, with sheer vertical walls of rock toward the 

 top. By degradation of the enclosing graywacke the dikes have been uncovered and 

 left bare. Mount Josephine, which stands near lake Superior and rises 703 feet above 

 its surface, is a specimen of these dikes, but it is inclosed in the slates rather than 

 in the graywacke. 



The gabbro intrusion extends thus to Pigeon point, where it is shown to be of 

 the same date as the great gabbro mass further west, by the effect it has on the 

 Animikie, producing the red rock of Pigeon point, the crystalline debris from which 

 is found in the Puckwunge conglomerate at the foot of Grand Portage island. The 

 same red rock is visible in some of the hills near Grand Portage village, also at the 

 east side of mount Josephine, and, according to Dr. J. G. Norwood, on the Pigeon river 

 where these dikes are exposed by that stream.* How much further east this igneous 

 intrusion extends, it is not at present possible to state. It is true, however, that the 

 same kind of topography and the same kind of sills in the Animikie with occasional 

 red rock areas, continue to Thunder bay and Silver islet. At the latter place red 

 quartz-porphyry pebbles are found in the conglomerate at the bottom of the sandstone 

 which lies non-conformably on the Animikie. Hence, it is reasonable to infer the 

 existence of quartz-porphyry in that region produced by the same cause as at 

 Pigeon point. 



The extension of the yabbro toward the west. The western limit of the gabbro is 

 represented, on our plates, to run southward across St. Louis county to the vicinity 

 of Short Line park, a few miles west of Duluth. But that is a boundary that is 



*DR. NORWOOD'S description and illustrations are on pages 407, 408, of OWKN'S report on the Grul/xju af Wimmxiit, luira ami 

 Minnesota, 18SS. 



