404 THE VERMILION IRON-BEARING DISTRICT. 



are found occasionally as accessory minerals, liypersthene, biotite, and 

 hornblende. 



Microscopic characters. — The gabbros were referred to above as being 

 generally distinctly granular rocks. In addition to the granular texture 

 one can recognize under the microscope the poikilitic and ophitic textures. 

 The poikilitic is the most common, and in rocks possessing this texture we 

 find plates of hornblende, augite less commonly, and biotite very rarely, 

 the poikilitic minerals including individuals of other minerals. The ophitic 

 texture occurs in the coarse gabbros, but is more common in the tiner- 

 grained forms. The presence of this ophitic texture in these rocks is 

 interesting as showing tlie gradations in textures from the A^ery coarse 

 gabbros to the finer-grained dolerites. The following are the original 

 constitutents which are still present in the gabbro: Augite, liypersthene, 

 olivine, a little bx'ownish-green hoi'nblende, biotite, apatite, and magnetite. 

 The plagioclase has been determined by Bayley" to be near basic labradorite 

 in character. These minerals do not possess any very exceptional characters. 

 Moreover, they have been described in great detail by Bayley'' in his 

 articles on the Duluth gabbro mass of northeastern Minnesota. The only 

 secondary mineral that has been observed is the serpentine. The rocks are 

 normally very fresh indeed. They vary considerably in mineralogic 

 composition. In some cases the feldspar is practically the only mineral 

 present, associated with only occasional grains of magnetite, and rounded 

 individuals of augite, forming anorthosite. With the feldspar occurs 

 occasionally a large amount of olivine and some titaniferous magnetite. 

 From the anorthosite phase the rock grades through facies containing tlie 

 ferromagnesian minerals in increasing quantity to the nearly ])ure titan- 

 iferous magnetite ores as the extreme variation. Occasionally^ the rock 

 consists of nearly pure au,gite with very little feldspar. Sometimes the 

 biotite is present in considerable quantity, producing the biotite-gabbro. 

 These are the varieties which we have observed. Many details concerning 

 these gabbros are given by Bayley" in the papers alread}' referred to. The 

 chemical composition of the gabbro is also here given. These analyses 



"The basic massive rocks of the Lake Superior region, by AV. S. Bayley: Jour. CTeol., Vol. I, 

 1893, p. 700. 



''The basic massive rocks of the Lake Superior region, by W. S. Bayley: Jour. Gaol., Vol. I, 

 1893, pp. 43,3-716; Vol. II, 1894, pp. 814-825; Vol. Ill, 1S9"5, pp. i-20. 



'■'Loo. cit., p. 712. 



