276 F. D. Adams — Melilite^bearing Rock. 



mum darkness is readied a slight revolution of the stage in one 

 direction will cause the pyroxene to assume a light brown tint, 

 while a movement of a few degrees in the reverse direction 

 will cause it to appear of a grayish-blue color, the section never 

 becoming black and the contrast of color often being very 

 marked. 



This same dispersion is seen in the pyroxene occurring in 

 the alnoite from Alno, in the melilite basalt from Hochbohl, 

 as well as in many of the nepheline bearing rocks found in the 

 vicinity of Montreal. It is believed to be a variety containing 

 a considerable amount of titanic acid but it has not as yet been 

 analyzed. Almost every one of the larger phenocrysts has a 

 narrow border or zone, well defined against the inner core, but 

 differing from it both in color and angle of extinction. This 

 zone, which can sometimes be seen even in hand specimens, is 

 light brown in color in the thin sections and probably has 

 the same composition as the pj'roxene occurring in the ground- 

 mass and forming the smallest phenocrysts, which have no 

 core, seeing that it resembles this in optical properties. This 

 zone has a larger extinction than the inner portion, the differ- 

 ence frequently amounting to 16° and even more. The crys- 

 talline outline of the pyroxene phenocrysts is frequently seen 

 to be much more perfect than those of the inner core, the lat- 

 ter often having a distinctly rounded form while the former 

 present true crystalline outlines. The zone moreover, although 

 sharply defined against the inner core along a curved line fre- 

 quently has a somewhat serrated outer limit showing like the 

 biotite many little projections running out into the groundmass. 

 The same phenomenon is noted by Tornebohm in the pyrox- 

 ene in the alnoite from Alno. It would thus seem that 

 the pyroxene like the biotite had first been crystallized out as 

 large phenocrysts, that these had been partially reabsorbed 

 giving to them rounded contours and that subsequently pyrox- 

 ene had again been crystallized out of the magma, forming the 

 zones about these larger phenocrysts, tending to restore to them 

 their original crystalline forms and also at the same time form- 

 ing the smallest phenocrysts and all the little pyroxene crystals 

 in the groundmass which, as above mentioned, resemble in 

 character that of the zones. 



The pyroxene occurring in the groundmass takes the form 

 of swarms of minute elongated often acicular crystals of a 

 light brown color. In some sections they constitute the greater 

 part of the groundmass. They have a high index of refrac- 

 tion, are not pleochroic and sometimes occur twinned parallel to 

 <xPx> . They occur very frequently penetrating the biotite 

 and other constituents of the rock and are often embedded in 

 them. 



