260 THE CRYSTAL FALLS IRON-BEAEINU DISTRICT. 



It will be seen from the analysis that the silica is somewhat too high 

 for the typical peridotites. This same fact is also emphasized by the 

 tendency manifested in some facies of the peridotite for feldspar to develop, 

 and thus for transitions to norite and gabbro to be produced. 



With this analysis of the peridotite there is placed for compa^rison the 

 analysis (No. 2) by Dr. H. N. Stokes of the picrite-porphyry already 

 described. The close resemblance chemically becomes at once manifest, 

 although the latter is more nearly a typical peridotite in composition. It 

 can not be denied that possibly this picrite is but a further differentiation 

 product of the same magma to which the peridotites belong, although its 

 occurrence is so remote from these that it is impossible to connect them in 

 the field. 



PERIDOTITE PROM SEC. 22, T. 42 N., R. 31 W., N. 1,990, W. 150. 



Just west of the northeastern corner of see. 22, T. 42 N., R. 31 W., 

 there is a bold outcrop of hornblende gabbro, which is cut by a dike, about 

 10 feet wide, of a very massive, coarse, granular black peridotite. Macro- 

 scopically one can readily distinguish in the peridotite flakes of biotite, 

 poikilitic plates of hornblende, and a smaller amoiint of white feldspar. 

 Under the microscope the constituents are, in order of importance: Horn- 

 blende, augite, feldspar, biotite, bronzite, olivine, magnetite, and quartz.^ 



Hornblende. — This Is tlic ricli browu kind, full of inclusions, grading into 

 the green variety which was described on ]d. 234 as occun-ing in the gabbros 

 of this district. It is present in anhedra inclosing biotite, pjn-oxene, and 

 olivine. 



Pyroxene. — Tliis is represented by monoclinic and orthorhombic varieties. 

 The monoclinic pyroxene, augite, is most abundant, and is in light-yellow 

 to pink-colored anhedra, except where it touches the feldspar; there the 

 augite is automorphic, and is surrounded b}' a narrow liorder of light- 

 lirown hornblende. 



The orthorhombic pyroxene is present in a few anhedra, which are 

 colorless or have a faint cream tint. It is presumed to be bronzite. 



Feldspar. — Tlils fiUs tlic iuterspaccs between the other constituents, and 

 occurs in grains which are poly synthetically twinned after the albite law. 



' Only one section has been prepared from this specimen, and it may not give a correct idea of 

 the true proportion of these minerals in the rock mass. In the macroscopical exaniinatiou of the 

 hand specimen the biotite seemed to be subordinate only to the hornblende. 



