GABBRO AND NORITB INTEUSIVES. 239 



of the district which crystalhzed before the pyroxene is oh-^ane. In the 

 peridotite, to be described in the next section, this is usually surrounded 

 by monoclinic or orthorhombic pyroxene. This altered mineral is not 

 important in quantity. 



Iron oxide. — Ilmeuite and titaniferous magnetite occur in some of the rocks 

 in considerable quantit}-. Both alter to spliene and rutile. 



Apatite. — Among the accessory minerals apatite is perliaps the most 

 common, and, as usual, one of the very earliest minerals to crystallize. It 

 is contained in all the essential constituents, and in biotite is surrounded by 

 pleochroic halos. In some cases it has even crystallized before sphene. It 

 is noticeable in some sections that great numbers of apatite crystals are 

 arranged along lines representing sections of planes between the plagioclase 

 plates, thus practically outlining the feldspar individuals. 



Sphene. — In mauy cases in these gabbros sphene is found contained in 

 some of the freshest rocks as an original accessory constituent. It is present 

 in largest quantity in the very finest-grained gabbros, which show a parallel 

 texture. In these rocks sphene in some cases surrounds an iron ore, which, 

 to judge from the rod-like sections which are so common, is ilmenite. One 

 might be led to think that the sphene was secondary in such cases, but the 

 iron ore is perfectly fresh, and, considei'ing that in the same thin section 

 crystals of apatite are also surrounded by sphene, it seems clear that we 

 may consider such sphene as original. It thus appears that a portion of the 

 titanium oxide combined with the iron oxide first, forming the titanic iron 

 ore. This was followed by the crystallization of the calcium-titanium 

 compound, thus giving the sphene. In these rocks sphene is not in crystals, 

 but in grains. .These grains are arranged in lon2" chains lying between the 

 other mineral constituents and Avith the long direction of the individual 

 grains, as well as of the lines of grains, parallel to the long directions of 

 the other constituents of the rock. 



Zircon and rutile. — Zlrcou is lu very small quantity. Rutile shows its usual 

 characters. It is most commonly associated with the octahedrite (anatase) 

 and brookite (?) as inclusion in the hornblende. The iron oxide is chiefly 

 present as ilmenite, with some titanic magnetite. 



The secondary minerals have already been mentioned and their char- 

 acters described under the description of the minerals from which tliev are 

 derived. 



