416 THE VERMILION IRON-BEARING DISTRICT. 



the first and second trestles east of Paulson's mine, there is a small mass of 

 crumpled, much metamorphosed black slate, with the main mass of the 

 gabbro to tlie south, and a small mass of gabbro to the north. This slate 

 is considered by Grant to be an inclusion in the gabbro. The writer is 

 inclined to think this slate is caught in the fork between the main gabbro 

 mass and a sill which is an ofiFshoot from it. Lack of exposures prevented 

 the tracing of the connection between them. It must be said in this 

 connection that in our work on the Vermilion iron district this relation 

 between the gabbro and the sills was considered a problem of importance 

 secondary to that of mapping the iron-bearing formations, and no especial 

 attempt was made to trace the relation between these rocks in the field. It 

 is believed that this connection would be shown to exist if the edge of the 

 gabbro and the adjoining Animikie area were mapped in detail. 



In this connection also it seems that the fact that the gabbro and sills 

 have not thus far been found in contact could be used as evidence more 

 strongly against the sills being older than the gabbro, as Grant" suggests, 

 since the gabbro is found in contact with every other rock in the district 

 which has been proved to be older than it is. If the sills are older than 

 the gabbro, they would form the one exception. 



8. Where the sills and the gabbro come nearest together the two rocks are 

 easily distinguished, even in the field. The few specimens about which there is 

 question have been, as far as examined microscopicallj', easily referred to one or the 

 other. 



Exception is taken to this statement, and the reader is referred to 

 section 4 above, where certain sills are mentioned in these words: 



In this connection it might be well to mention some sills in the Animikie at 

 Akeley Lake in the Akeley Lake plate; these are apparentlj' of gabbro. They are 

 fine-grained at their edges, but even here the structure is more nearlv that of the 

 gabbro, and not that of the ordinary sills. 



The rock of the sills can in general be readily distinguished from that 

 of the main gabbro mass, but so can the dike rocks of rhyolite, etc., from 

 the coarse granite of the massives from which demonstrably the dikes are 

 offshoots. This difference is not evidence of importance against the rela- 

 tionship of the sills and of the gabbro. 



«Geol. and Nat. Hist. Survey of IMinnesota, Final Kept., Vol. IV, 1899, p. 488. 



