412 THE VEEMILION IRON-BEARING DISTRICT. 



writer is prepared to go much farther than Lawson, and to express the 

 opinion that these sills are the* equivalent in age of the Duluth gabbro of 

 northeastern Minnesota, and even that they were derived from the same 

 magma from which it was derived. In the following paragraphs Grant's 

 statement, as quoted above, will be discussed and evidence will be presented 

 to support the above-mentioned view of the intimate relationship of the sills 

 and gabbro. This evidence was collected partly during work on the Ver- 

 milion district and during several reconnaissance trips into the Keweena- 

 wan gabbro from the Vermilion district, in the course of a portion of a 

 season's field work on the Keweenawan of Minnesota and Canada, made in 

 1900. 



No. 1. The sills are considei'abh^ altered, i. e., the pyroxene has usually largely 

 been replaced by secondary minerals, while the gabbro is usually fresh and the 

 olivine as well as the pyroxene is usually unaltered. 



The facts are essentially con-ect, and no explanation can be ofPered for 

 this difference, unless it is that it results from the sills being of smaller 

 mass and intercalated in the slates and having been exposed in consequence 

 to a more energetic action of water than has the gabbro. The gabbro 

 disintegrates very easily, and even when the state of aggregation is such 

 that the rock can be easily crushed in the hand the constitutents are rela- 

 tively fresh. This gabbro, as a result of this readiness to disintegrate, has 

 had its outer disintegrated portion removed by glacial action and water 

 erosion. In general, erosion has kept pace with the disintegration of the 

 gabbro. Hence the rock which we now observe is very fresh. The sills, 

 on the other hand, are very much more resistant. All those examined 

 were fairly fresh and exceedingly hard and tough. If one could get a 

 specimen from the rocks of the sills deep down in the mass, doubtless the 

 rock would be found to be about as fresh as the gabbro. 



2. The sills are essentially nonolivinitic; at least traces of olivine, even when 

 altered, are not common. The gabbro is normally olivinitic. 



This is also true. But one must not neglect the fact that locally the 

 gabbro is also practically free from olivine. The local absence of olivine 

 in the gabbro is due to conditions of crystallization of the gabbro magma 

 and possibly slight chemical differences also, and just in the same way can 

 one explain the absence of olivine from the sills. No analyses have been 



