21 



The chief of the dark coloured constituents, which are 

 usually grouped together, is a very deep brown hornblende. 

 A few scales of muscovite and rare fragments of pyroxene 

 enclosed in hornblende were noticed. Magnetite and 

 apatite are conspicuous associates of the hornblende and 

 biotite. Nepheline, usually decomposed to hydronephelite. 

 is sometimes present in very small amount. 



Nepheline syenite. — The nepheline syenite may in 

 a general be described as a medium grained rock of granitic 

 habit varying from pale grey to dark grey in colour. 

 Many outcrops are pinkish or purplish owing to the relative 

 abundance of hydronephelite, a decomposition product 

 of the nepheline. When present in very considerable 

 amount, as is often the case, it produces a striking and 

 beautiful rock. Gneissoid structure is very uncommon, 

 but occasionally a peculiarly banded structure, due to 

 the segregation chiefly of the darker coloured minerals, 

 is in evidence. Weathered surfaces are characteristically 

 pitted owing to the rapidity with which nepheline de- 

 composes. 



The most abundant mineral constituent is feldspar. 

 Nepheline sometimes constitutes one-sixth of the whole 

 rock mass (hill east of Coldwell station). Hydronephelite 

 is always present, while hornblende and magnetite and the 

 less abundant pyroxene are also readily distinguishable. 

 In most instances the coloured constituents are very 

 subordinate, but in some cases they form the bulk of the 

 rock. 



All the feldspars belong to the natron-orthoclase- 

 microperthite series. All gradations from undoubted 

 pure natron-orthoclase to distinct microperthitic inter- 

 growths of orthoclase and albite are found. Nepheline 

 is always the last mineral constituent to crystallize, occupy 

 ing the irregular interspaces between the other constituents. 

 As a rule it is usually decomposed in part, or altogether, 

 to hydronephelite. This orange-red hydronephelite is 

 the most striking mineral constituent of the nepheline 

 syenite. It is undoubtedly the orange-coloured nepheline 

 of the original descriptions by Logan. This mineral is 

 very abundant and characteristic. It occurs both in 

 simple individuals, often of microscopic dimensions and 

 sometimes with centres of unaltered nepheline still 

 remaining, and also in aggregates of several individuals 

 up to half an inch (1-27 cm.) or even more in diameter. 



