THE KEWEENAWAN. 413 



made of the various facies of the gabbro to determine the chemical differ- 

 ences which may exist between them, nor have analyses been obtained of 

 the rock of the sills to prove its identical chemical composition with the 

 gabbro as a whole or with any of its special facies. 



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

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

 ferromagnesian minerals, and the rock is light gva.y in color. When a basic mineral 

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



The writer does not concur in this statement. The fact that the sills 

 are generally darker than the gabbro is due largely to the finer grain of the 

 sill rocks. When, however, the sills are very coarse grained one finds 

 patches that are made up almost exclusively of feldspar in large individuals 

 and such areas in the sills are a very light gray and become, when the feldspar 

 is kaolinized, nearly snow white. On the other hand, some of the sections 

 from the sills which have been examined are made up largely of magnetite, 

 with pyroxene second in abundance, and last, the feldspar. This proportion 

 of minerals is not the rule in the sills, but neither is it the rule in the g'abbro, 

 as witness the light-gray anorthosite with but little of the ferromagnesian 

 compounds. 



4. 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 Akeley Lake, in the Akeley 

 Lake plate; these are apparently 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. 



The writer must disagree with the above statement of the texture of the 

 rocks, as his own studies have shown that while the gabbro is predomi- 

 nantly granitic, nevertheless the ophitic texture is very frequent in the 

 finer-grained facies. On the other hand, the sills show, in places where the 

 rock is as coarse as is the finer-grained gabbro, a distinctly ophitic texture, 

 grading into an imperfectly granular one, and from this down into very 

 fine-grained intersertal textured basalts. A porphyritic texture which was 

 not observed in the gabbros is common in the sills. These facts indicate a 

 textural gradation between the gabbro and sills, the differences in general 

 textural characters being due to the difference in the conditions between 

 the slow crystallization of an enormous mass of magma, as in the case of 



