CHARACTERISTICS OF THIRD VARIETY OF GABBRO. 273 



have been glaciated, but the cleavage faces of the large feldspars are 

 plainly seen. Other minerals, as a rule, are not determinable, but at 

 some points there are considerable quantities of magnetic iron ore. 



At most outcrops careful inspection shows a certain amount of ])aral- 

 lelism in the arrangement of the feldspars. On the one hand this is 

 very obscure, while on the other the rock becomes distinctly gneissoid. 



Microscopic Cluiractcrisiics and mineralogic Composition. — ^licroscopic 

 sections show that the rock, as indicated in the hand specimens, consists 

 chiefly of feldspar. The more typical specimens have cores of well 

 twinned plagioclase, surrounded by a margin usually composed of 

 microperthite. In such cases the fine tongues and spindles of the 

 microperthite correspond optically with the material of the core, while 

 the inclosing feldspar is more acid, as indicated by its lower interference 

 colors. The basic character of the core accounts for its more ready dis- 

 integration, as seen in the field, while its dark color is explained by the 

 presence of inclusions of magnetite. 



The same microsections generally show other feldspar individuals con- 

 sisting wholly of microperthite, and comparison of different sections 

 shows that the microperthite may increase till it becomes the only feld- 

 spathic constituent. 



The low extinctions and interference colors of the feldspars, as well as 

 a mean index of refraction lower than that of quartz, as measured by 

 Becke's method, show that the plagioclase is near the albite end of the 

 series. The high percentage of soda shown by the analysis points to the 

 same conclusion. Anorthoclase may well be preeent, but its separation 

 has not been attempted. 



Of ferro-magnesian constituents, monoclinic pyroxene is largely pre- 

 dominant, hornblende filling a very minor role and biotite rarely present. 

 The pyroxene is usually deep green and non-pleochroic. The diallagic 

 })arting is absent, though a fi])rous structure is sometimes shown. The 

 mineral is always in irregular grains, and usually in small amount as 

 compared with the feldspar. Magnetite is generally seen, ranging from 

 the small inclusions in the feldspar w\) to irregular masses of considerable 

 size. These ma}' constitute so large a proportion of the rock as to render 

 it a considerable ore body. Slender prisms of apatite are not uncommon. 

 Quartz also appears in a number of sections, but there is reason for be- 

 lieving that some of it is secondary. Other constituents of minor impor- 

 tance are more common in certain modified i)()rtions of tlie rock. They 

 will ])e referred to in the sc(iuol. 



The most striking feature shown under tlic microscope is a ])eautirully 

 developed cataclastic structure. This is never lacking, though varying 

 in ditferent specimens. When least marked, the feldspars are separated 



