SOME UNUSUAL ROCKS FROM MAINE 



175 



become more and more elongate until we have a single long, needle- 

 like crystal which resembles the rutile needles common in many 

 quartzes. In certain of the feldspars these inclusions are arranged 

 in two or three intersecting sets which, in some cases at least, par- 

 allel the cleavage planes, though in other parts of the feldspar they 

 seem to have every conceivable orientation. Besides those men- 

 tioned above, there occur, scattered irregularly through the feld- 

 spar, a large number of minute, globular, or prismatic inclusions 

 of indeterminate character. 



The zonal structure observed in many of the feldspars is the 

 result of the alternation of bands of only slightly different com- 

 position, rather than any considerable and progressive change in 

 composition in passing from the center outward. The index of 

 refraction in this case never rises above that of Canada balsam. 

 Some bands are orthoclase, while others are microcline, and sHght 

 variations in composition are also indicated by slight differences 

 in the double refraction and in the extinction angles. Inclusions 

 are very much more abundant in some of the bands than in others, 

 and it is those bands which contain the abundant inclusions that 

 are dark- colored megascopically. This suggests that the peculiar 

 purplish-gray color of certain parts of the feldspar may be due 

 largely, if not wholly, to the inclusions. 



The groundmass, when examined microscopically, shows the 

 minerals indicated in the following table: 



Biotite 



Hornblende 



Titanite 



Apatite 



Quartz 



Titaniferous Magnetite . 

 Feldspar 



Total. 



*An estimated amount of i per cent, of quartz occurs as inclusions in the feldspars. 



Brown biotite, the most abundant of the ferromagnesian con- 

 stituents, occurs in plates of various sizes up to 2™™ across. All 



