878 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



tions (rhoraben-porphyr). Twinning is on the Carlsbad plan, and 

 the two cleavages are nearly at right angles to one another. In 

 addition a fine striation due to polysynthetic twinning is readily 

 observed. This felspar is a microcline microperthite (or anortho- 

 clase microperthite). Often cloudy microcline (moirirten-micro- 

 clin) and gitter microcline are abundant constituents. The base 

 consists in these rocks essentially of anorthoclase. These soda- 

 potash felspars, so common in the Christiania region, are extremely 

 abundant and well developed in the Nandevvar alkaline rocks, 



Arfvedsonite is rare in these rocks, bat occurs sometimes. 



jEgirine is occasionally present, but more often it is represented 

 by an segirine-augite or light green diopside (salite). Olivine is 

 never present, and in fact the analyses display so low a magnesia 

 percentage that all this constituent must be incorporated in the 

 segirine-augite (see Analyses, Table i. and Calculations of the 

 norm. Table ii.). The main bulk of these rocks consists invariably 

 of felspar, the other constituents never attaining great abundance. 



As minor constituents we find nepheline (in the groundmass), 

 decomposition-products after nepheline such as cancrinite, kata- 

 pleiite, geisikite (?), liebnerite (?) and zeolites; also apatite, fluor- 

 spar, zircon, ferrite, magnetite, haematite, ilmenite. Chlorite, 

 secondary after segirine-augite, is a common constituent in many 

 of the darker porphyrites. Occasionally sodalite is present. 

 Quartz may also occur. 



iii. The post-trachytic basic lavas have many similarities with 

 the phonolitic and other intermediate lavas, in texture, mineral 

 composition, and chemical composition. They are poor in 

 magnesian minerals, olivine being usually absent (cp. the basalt 

 of the Sandilands Ranges, New England). 



iv. The basic rocks of greater antiquity than the trachytes, 

 namely the dolerites intruding the Permo-Carboniferous strata 

 at Bullawa Creek and Bobbiwaa Creek, form a distinct group. 

 The pyroxene is a deep brown highly pleochroic titaniferous 

 augite accompanied by a diallage. Olivine is abundant, and the 

 felspar is very calcic. Some of these rocks, however, contain 



