THE KEWEENAW AN. 41 I 



as evidence " ag-ainst the relationship of the two kinds of rocks he summarizes 

 as follows: * 



1. The sills are considerably altered, i. e., the pja-oxene has usuallj- largely 

 been replaced by secondary minerals, while the gabbro is usually fresh and the 

 olivine as well as the pyroxene is usually unaltered. 



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

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



3. The sills are quite rich in ferromagnesian minerals, giving a dark-gra}' or 

 black color to the rock. The gabbro is usually rich in feldspar and rather poor in 

 ferromagnesian minerals, and the rock is light gra}' in color. When a basic mineral 

 predominates, it is mostly iron ore, which is not the case with the sills. 



i. The sills are in structure ophitic; the gabbro is granitic. This holds true 

 also of the coarsest-grained sills and of the finest-grained gabbro. In this connection, 

 it might be well to mention some sills in the Animikie at Akelej' Lake in the Akeley 

 Lake plate; these are apparentl}" of gabbro. They are fine grained at their edges, 

 but even here the structure is more nearly that of the gabbro, and not that of the 

 ordinary sills. 



5. The sills are verj' fine grained, almost glassy at the lower and upper sides, 

 even in the thickest sills. The gabbro is not verj- fine grained at the contact with 

 the Animikie rocks, and even on the edges of the apparent gabbro sills mentioned 

 above, the fineness of the grain nowhere a^Dproaches that of the edges of the ordinary 

 sills. 



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

 rocks for onlj' a very few feet, or even inches, from the contacts, while the 

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

 distance of several rods. 



7. The gabbro and sills have not been traced together, neither have they 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 ma}', of course, come into 

 actual contact with the gabbro. 



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 microscopically, easily referred to one or the other. 



In closing his summary Grant says that he is inclined to the idea that 

 these sills are of earlier date than the gabbro. 



Several years before the publication of the above statement Lawson 

 expressed his opinion '' that the sills are identical with many of the heavy 

 sheets of dark diabase and gabbro which prevail on the eastern part of the 

 Minnesota coast, and which are associated with the Keweenawan. The 



«Geol. and Nat. Hist. Survey of Minnesota, Final Eept., "Vol. IV, 1899, p. 488. 

 6 Geol. and Nat. Hist. Survey of Minnesota, Bull. No. 8, 1893, p. 47. 



