THE KEWEENAW AN. 419 



hypersthene, augite, brownish-green hornblende, and magnetite, and 

 possibly the reciystallization of the feldspar. 



Lower Huronian. — In the Lower Huronian sediments this production 

 of biotite has been accompanied by a recrystallization of the minerals of 

 the rock, whereby the sedimentary structures are mostly destroyed and 

 mica-schists and gneisses are produced. Immediately along the edge of 

 the contact with these schists in the metamorphosed sediments occur large 

 quantities of ferromagnesi'an minerals, such as augite, hypersthene, and 

 brown hornblende. In the case of the conglomerates we find, since there are 

 a number of difi"erent kinds of pebbles, and these differ from the matrix in 

 which they lie, that the pebbles when aflPected furnish a somewhat different 

 product from the matrix, and this product usually stands weathering better 

 than the matrix, so that the pebbl)^ character of the conglomerate is retained. 



Upper Huronian (Gunflint formation). — Where the gabbro has been in 

 contact with or very near the rocks of the iron-bearing Gunflint formation, 

 it. has affected them in a very marked way, and has produced magnetite 

 ores interlaminated with bands of quartz and a rock composed of olivine, 

 hypersthene, augite, hornblende, and magnetite, with quartz in varying 

 proportions. The rocks thus produced are different from any original 

 igneous or sedimentary rocks with which the writer is acquainted. 



Bove slates. — The metamorphism of the Rove slates by the gabbro 

 has been such as to produce, from rocks consisting of angular grains of 

 quartz, feldspar, and plates of chlorite, and a dark interstitial material, 

 more or less completely crystallized mica-schists, or, with larger jDercentage 

 of feldspar, rnica-gnieisses, if these terms can be applied to rocks that have 

 an essentially granitic texture, although the original banding of the sedi- 

 ments remains in the recrystallized rock and causes it to break more readily 

 along these bands than in any other direction. 



The action of the sills is ver}^ slight upon the slates, in most cases 

 producing rocks which are mica-schists, but not so completely recrystallized 

 as in the case of those affected by the gabbro. 



Endomorphic action — There is, along the margin of the gabbro, a 

 somewhat finer grained facies than further in the mass. The sills also 

 show distinct selvages, but neither the gabbro nor the sills appear to have 

 had their textures or general characters otherwise modified as a result of 

 contact with the various rocks mentioned. 



