STRUCTURAL GEOLOGY. 61 



The Grand Portage dikes.] 



6. They pass from one horizon to another. 



7. The bottom of the sedimentary strata above them, wherever it is observable, is a freshly ruptured surface. 



8. Apophyses of the trap pass from the main sheet into the cracks of the slate above and below. 



9. The trap sheets, particularly at the upper contact, hold included fragments of the overlying slates. 

 10. They locally alter the slates above and below them. 



Besides the sills, which date from the gabbro, there may be others of later date, 

 and to these will apply the argument of Lawson, that they are younger than the 

 Keweenawan. But no data are as yet at hand to warrant an attempt to separate 

 such sills from those of the age of the gabbro. 



The Grand Portage dikes. There are at least two systems of dikes in the vicinity 

 of Grand Portage. This is attested not only by the observations of this survey, but 

 also by those of Norwood, who examined this region for the Owen survey. The 

 general trend of the most of the great dikes is toward the northeast by east, but 

 that which forms Hat point, culminating in mount Josephine, is nearly at right 

 angles with that direction. Much of the rock in the mount Josephine hill, south 

 from the summit, has a basaltic columnar structure, the columns standing vertical, 

 indicating that a dike here is merged into a sill. Several of these great dikes, which 

 are several hundred feet wide, and sometimes rise above the adjoining slates with 

 vertical walls from 50 to 100 feet, are noticeably crossed by a series of smaller dikes 

 running nearly at right angles to them. 



The individual great dikes cannot be assigned to different dates except by some 

 careful field examination more detailed than it has yet been possible to give the 

 region. Yet it is evident from general considerations that some are older than the 

 others, for they produced the red rock which has supplied debris to the Puckwunge 

 conglomerate, while the younger cut some of the amygdaloids which lie upon that 

 conglomerate. Some of the latter can be seen on Grand Portage island and on the 

 points west from Grand Portage bay. There is an interesting problem connected 

 with these dikes which must be left for the future student to solve. The petrograph- 

 ical study which has been given these dikes seems to afford no criterion for discrim- 

 inating them, for some of the younger, as well as the older, present the augite 

 cotemporary with or earlier than the plagioclase, a characteristic which is sometimes 

 said to belong to gabbro. 



Effect on the Animikie the red rock. It has already been stated, in describing 

 the strike of the Animikie rocks, that the Animikie formation is completely lost, 

 as such, in the gabbro mass. Observations made on Pigeon Point peninsula at mount 

 Josephine, on one of the islands of the Lucille group (south of Pigeon point), at 

 Duluth, at Brule lake, have now well demonstrated that the Animikie is converted 

 into the "red rock," so called, which extends from the place at which the Animikie 

 disappears in the "gabbro flood," with a nearly continuous surface band to Duluth.* 



*The phenomena at Pigeon point have been discussed by W. S. BATLKY in a bulletin of the U. 8. Qeol. Survey, viz. , No. 109, 1 



