MANSFIELD AND LARSEN : NEPHELINE BASALT 465 



specimen it shows abundant pale yellowish green grains of olivine 

 up to 2 millimeters across and a few small grains of augite in an 

 aphanitic groundmass. It contains a few small inclusions of 

 shaly material and one large inclusion of a nearly white, granu- 

 lar rock made up almost entirely of striated feldspar and quartz. 



Microscopic examination shows the rock to be made up largely 

 of pyroxene and olivine with some nepheline, phlogopite, iron 

 ore, and apatite. The texture is porphyritic and the pheno- 

 crysts, which average about a millimeter in cross section, and 

 are chiefly forsterite with some diopside, make up about a third 

 of the rock. They are imbedded in a holocrystalline ground- 

 mass made up chiefly of rods and grains of diopside which lie in 

 a clear matrix or are poikilitically included in phlogopite. The 

 clear matrix is probably in part nepheline but may contain some 

 glass. The average cross section of the minerals of the ground- 

 mass is about 0.01 mm. 



The forsterite is in clear grains of the usual form. In cross 

 sections the grains range from over 2 millimeters to a small 

 fraction of a millimeter, but the greater part are not far from a 

 millimeter. They have the following optical properties: « = 

 1.641 ±0.003; /? = 1.661 ±0.003; t = 1-680 ±0.003; 2V =87° (com- 

 puted from the refractive indices) ; optically + . In convergent 

 light the bars of the interference figure are nearly straight and 

 the dispersion of the optic axes is barely perceptible with p< v. 



In addition to embayments filled with the groundmass the 

 forsterite carries a few inclusions of the iron ore and chains of 

 gas or liquid inclusions. The mineral shows no signs of altera- 

 tion except for a narrow reddish border which probably repre- 

 sents incipient alteration to iddingsite. A Rosiwal determina- 

 tion on two thin sections showed 24.9 ± 1 per cent by volume 

 or 26.4 per cent by weight of olivine. 



The diopside is mostly in the groundmass, but the crystals 

 vary greatly in size and some are a millimeter across. The diop- 

 side is nearly colorless but shows a faint greenish cast as com- 

 pared with the water-clear olivine. A very few of the larger 

 crystals show small cores or narrow zones which are rather deep 

 green and pleochroic. It shows zonal growths with somewhat 

 different extinction angles and a faint hour-glass structure. The 



