250 THE CRYSTAL FALLS IRON-BBAKIXG DISTRICT. 



A rock to be described last, connecting the diorites, gabbros, and peridotites, 

 was taken from near the northeast corner of sec. 22, T. 42, B. 31, where it 

 is found cutting the gabbro. 



PETROGRAPHICAL CHARACTERS. 



The peridotites are very dark green to black coarse-grained rocks, 

 showing in almost all cases a granular texture. In one case an excellent 

 poikilitic texture was observed, and in another the same texture in a very- 

 imperfect condition was seen. The almost total absence of any pressure 

 phenomena in these rocks excludes the idea of their having been subjected 

 to any powerful dynamic action. The chief mineral constituents, arranged 

 in order of relative amounts, are pyroxene (monoclinic and orthorhombic), 

 olivine, hornblende, and biotite. The following minerals also occur associ- 

 ated with these: Feldspar, apatite, green and brown spinel, and iron oxide. 



Pyroxene — There Is prescnt in these rocks both orthorhombic and mono- 

 clinic pyroxene. The orthorhombic has a yellowish tinge, and contains a 

 few tabular inclusions. Only a few grains of this pyroxene were found, 

 but upon one section a figure was obtained and the positive character of the 

 mineral Avas determined. It appears to be bronzite. The bronzite grains 

 are found included in the hornblende. Its relations to other minerals are 

 not shown in any of the thin sections. 



The monoclinic pyroxene, though one of the earliest minerals to crys- 

 tallize, is likewise in xenomorphic individuals, many of them twinned, some 

 poly synthetically. It is usually of a light yellowish color, and is then 

 nonpleochroic; in some sections, however, it is sufiiciently colored to show 

 pleochroism from light yellowish to bi'ownish. It includes a number of the 

 clove-brown tabular interpositions like those in the hornblende, and these 

 at times give the pieces a decided violet tinge. Less commonly than the 

 tabular interpositions one observes green needles and laths, more rarely 

 rounded grains or plates of larger size and having a brownish-green color. 

 They are so minute as to defy positive deterininatioii, but are presumed to 

 be hornblende microlites. The orthopinacoidal parting is in some cases 

 well developed in the augite. This diallagic augite is in large quantity, 

 and though usually included in the hornblende, nevertheless includes both 

 hornblende and biotite in a few ragged plates. Such sections are pi'obably 

 those which have passed through the outer edges of the augite crystals. 



