LARSEN AND HUNTER: MELILITE 475 



1.6326, av. 1.6323; e^, = 1.6254 and 1.6259, av. 1.6257; 

 -WNa — fxa = 0.0066; (w — e) measured = 0.007. In medium- 

 grained material from another locality, ws-a = 1.6327 and 1.6336, 

 av.J.6331; 6.,= 1.6270 and 1.6273, av. 1.6271; (co.. - e.va) = 0.0060. 

 In fine-grained material from a third locality, w.va = 1.6327; 

 exa = 1.6258; coxa — €Na = 0.0069. Repeated measurements on 

 a single plate checked to within 0.0002 so that the differences 

 shown above are due to an actual difference in the values for 

 different fragments. 



In the greater part of the thin sections examined the melilite 

 shows normal grayish interference colors ahnost identical with 

 those of the associated apatite. In a few it shows abnormal 

 blue colors and has a birefringence considerably lower than that 

 of the associated apatite. 



Resemblances. In the hand specimen the melilite closely re- 

 sembles nephelite and, indeed, it is not always possible to dis- 

 tinguish between the melilite of some of the melilite rocks and 

 the nepheline of some of the associated ijolites without the aid 

 of a microscope. It is also difficult to distinguish the melilite 

 rocks from some of the associated apatite-rich rocks, and chemi- 

 cal tests are necessary in some cases as the two minerals are 

 similar both megascopically and microscopically. The apatite 

 is whiter than the melilite and in thin sections it lacks cleavage 

 and is commonly clearer than the melilite; otherwise its optical 

 properties, when in granular aggregates, are almost identical 

 with those of melilite. 



Chemical properties. Material for the chemical analysis was 

 carefully picked by hand from one of the coarse-grained speci- 

 mens collected from an outcrop about 100 yards northeast of 

 the point at which the mapped road up the south fork of Beaver 

 Creek crosses that creek and about a mile above the mouth of 

 North Beaver Creek. A careful microscopic examination of the 

 sample showed the presence of a small amount of calcite and 

 apatite, still less of pyroxene, and a very little opacite as small 

 inclusions in the melilite. The mineral gelatinizes readily when 

 treated with weak acids. 



