GABBRO AND NOEITE INTRUSIVES. 241 



The coarse-grained porphyritic gabbro forming the greater part of the 

 knob consists of plagioclase, hornblende, biotite, and iron oxide, with a very 

 small amount of jjyroxene. The hornblende occurs in phenocrysts which 

 have irregular rounded shapes instead of being well crystallized. Some of 

 the largest phenocrj^sts have a diameter of slightly less than 1 centimeter. 

 They are poikilitic, rendered so by inclusions of lath-shaped plagioclase and 

 rounded grains of pyroxene. (Photomicrograph, fig. A, PI. XLI.) This por- 

 phyritic hornblende is a dark reddish-brown variety containing such great 

 numbers of minute inclusions as to be opaque in many places, which grades 

 over into, and is in many places in optical continuity with, a dirty green 

 hornblende. This green hornblende is in anhedra and forms the cement 

 for the feldspar, and the two together the groundmass for the brown horn- 

 blende phenocrysts. The plagioclase is most commonly in broad, well- 

 developed crystals, which frequently give quadratic sections. Some few 

 grains of a pink monoclinic jjyroxene are included by the hornblende. 



SBCS. 15, 22, 2S, AND 29, T. 42 n., r. 31 w. 



Exposures of a hornblende-gabbro with interesting facies associated 

 with it occur in the southeastern corner of sec. 15, at the southeastern corner 

 of sec. 22, extending east and west through the northern part of sec. 28, at 

 the southeastern corner of sec. 28, and on the west bank of the Michigamme 

 River in sec. 29, T. 42 N., R. 31 W., at the location N. 100, W. 1,250 paces. 

 This is medium to coarse grained and of a gray color from a short distance. 

 Examined at moderately close quarters, one distinguishes very readily a 

 milky white feldspar and a black or dark-green hornblende in about equal 

 quantities. The microscopical examination adds to these two minerals in a 

 very subordinate quantity biotite, pyroxene, and orthoclase. The labra- 

 dorite plagioclase is in medium-broad, irregular plates, though at times 

 approaching a very distinctly lath-shaped form. The orthoclase is present 

 in a few rare individuals in the form of irregular plates. The hornblende 

 constituent is in irregular plates and varies in character. It may be the 

 brown or the green variety already described, or the two together in sepa- 

 rate individuals, or even the brown grading into the green. This green is 

 original and not the alteration product of the brown. Biotite is the normal 

 reddish-brown kind in irregular plates. The pyroxene is usually absent 



MON XXXVI 16 



