PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 83 



Unliliro.] 



measurement is near the locus of an optic axis, and in auorthite is far removed from 

 it, search must be made for the proximity of the axial figure ; the feldspar is thus 

 again found to be labradorite. 



Micro-chemical tests (Boricky) show the presence of lime and soda in this 

 feldspar. 



The specific gravity of small fragments of the feldspar from No. 1 was found 

 to be 2.706. The trial was made in iodide of methyl on a Westphal balance. 



The impurities of this feldspar, as exhibited in the section, tend to accumulate 

 in lines or elongated masses parallel with the cleavage planes. They are more 

 abundant in proximity to the other minerals, and especially adjacent to the pyroxenes, 

 where an opaque whiteness often spreads irregularly for some distance on all sides, 

 sometimes tinged with green. The same cloudiness accompanies all other cracks, 

 which sometimes may be seen crossing the feldspars irregularly, and occasionally it 

 pervades generally the mass of the feldspar, destroying the characteristic optical 

 phenomena. In addition to this kaolinization are minute grains of magnetite (?) 

 which are in groups or scattered. Occasionally they afford a cubic section, but 

 generally their outlines are irregular. The pyroxene, which ophitically embraces 

 the feldspars, is frequently, and perhaps usually in the specimens from Duluth, much 

 decayed. The brilliant polarization which this mineral, when pure, presents between 

 crossed nicols, in thick sections prepared for the microscope at low powers, is not 

 usually seen, at least in sections cut from No. 1; but owing to incipient disintegra- 

 tion secondary products obscure it. These consist of magnetite, amphibole and 

 chlorite, and sometimes a small amount of epidote. 



Chemical analysis of the feldspar of No. 1. The following analysis was made by Prof. J. A. Dodge. He 

 selected the feldspathic portion, by mechanical means, as clean as possible, but this portion was not wholly 

 unmixed with other constituents of the rock : 



SiO 2 49.78 



CaO 11.55 



MgO 1.43 



K 2 .41 



Na 2 O 339 



H,O - 1.83 



100.76 



The pyroxene was formed after the feldspars, filling the irregular, angular spaces 

 between them, producing the ophitic structure. Usually the magnetite which has 

 resulted from the breaking down of the pyroxene is in minute grains intimately 

 associated with the resulting amphibole, but grouped together near the centre of the 

 original pyroxene grain or in zonal arrangement at the periphery. But sometimes 

 the amphibole occupies the entire original space of some of the pyroxenes, while 

 magnetite occupies others. In each case the outlines of the pyroxene are preserved. 



*The amount of oxide of iron is quite small. 



