466 



MANSFIELD AND LARSEN: NEPHELINE BASALT 



extinction angles are very large; those for the green part are the 

 larger, indicating a higher content in iron. 



The nepheline is interstitial to the diopside. It is clear, has 

 an index of refraction near that of Canada balsam, and is indis- 

 tinctly polarizing. It is commonly without crystal outline but 

 square sections are occasionally present. It is in too small 

 grains for positive identification by optical means alone. Some 

 glass may be present; no feldspar was found. 



In elongated streaks or irregular patches the abundant diop- 

 side rods are embedded poikilitically in mica, the mica forming 

 less than half of the area. This mica is pleochroic, rather light 

 reddish brown parallel to the cleavage and nearly colorless 

 normal thereto. It is probably phlogopite or a biotite rather 

 poor in iron. 



Chemical composition. An analysis of the nepheline basalt, 

 inade by W. C. Wheeler in the laboratory of the Geological Sur- 

 vey, is given in column 1 below and is accompanied by the analy- 

 ses of similar rocks for comparison: 



Analyses of nepheline basalt and related rocks. 



Si02... 



AI2O3. 



FeaOs. 



FeO.. 



MgO.. 



Ca.O. . 



NaaO. 



K2O.. 



H2O- 



H2O+ 



TiO.. . 



CO.... 



P2OS.. 



S 



Cr203. 

 NiO.., 

 MnO.. 

 BaO. . 

 SrO... 



* Including V2O3 0.04, F 0.07, and deducting 0.030 for F. 



