THE KEWEENAW AN. 415 



This difference in grain of the rock on the edge of the sill and of that 

 on the edge of the main mass of the gabbro is the difference which normally 

 occurs where there is such a great discrejjancy in the size of the masses as 

 there is in the case under consideration. Witness, for example, the character 

 of the rock at the edge of any very large granite massive and that upon 

 the edge of the dikes radiating from it. Undoubtedly, however, the gabbro 

 is materially finer grained on the periphery of the mass than it is in the 

 interior, and there is an imperceptible gradation between the fine and 

 the coarse facies. Moreover, the border facies of the gabbro is about of 

 the same degree of coarseness or possibly even less coarse than the rock 

 occurring in the interior of the thickest sills. From the nature of the 

 occurrence one should not expect the border of the gabbro to be as fine as 

 the rock upon the edge of the sills. 



6. The sills, even the largest ones, have macroscopically altered the Animikie 

 rocks for only a very few feet, or even inches, from the contacts, while the meta- 

 morphism of the Animikie at the gabbro contact is profound, extending for a distance 

 of several rods. 



The relative intensity of the metamorphic action of the gabbro and sills 

 depends largely, as does the size of the grain of the rocks, upon the masses 

 of the magma, since this influences the rate of cooling. Of necessity a small 

 mass of rock like a sill would have less effect ujjon the sediments than 

 the gabbro. 



One is better prepared to appreciate the difference in the effect of the 

 sills and main gabbro mass when one thinks that the thickest sills are only 

 about 400 feet thick, and that these are utterly insignificant when compared 

 with the Duluth gabbro mass which covers about 2,400 square miles in 

 Minnesota." 



7. The gabbro and sills have not been traced together; neither have the}^ been 

 found in contact. In the map the sills have not been shown in contact with the 

 gabbro; this is on account of lack of exposures. They may, of course, come into 

 actual contact with the gabbro. 



It is true that no actual contacts of gabbro and sill have been 

 observed, although they have been seen separated only by a short distance. 

 At one place on the Duluth, Port xlrthur and Western Railroad, between 



«The geology of the Keweenawaii area in northeastern Minnesota, III, Pt. II, Geology of the 

 Keweenawan series, by A. H. Elftman: Am. Geologist, Vol. XXII, 1898, p. 132. 



