120 Transactions. 



section of grey augite, surrounded by a mass of allotriomorphic crystals 

 of lime-soda feldspar and grains of magnetite. 



Groundmass. — The groundmass consists of lath-shaped feldspars, pale- 

 green augite, numerous magnetite grains, and interstitial nepheline. 



The feldspar is chiefly orthoclase or sanidine, but oligoclase also occurs, 

 with the same characters as in the phenocrysts. The feldspar laths vary 

 in size up to 0*5 mm. in length and 005 mm. in breadth. 



The pale-green ;egirine-augite occurs in irregular crystals. It has the 

 same characters as in the phenocrysts. The pleochroism is very slight. 

 The maximum size of the crystals is 0-5 mm. by O'l inm. 



Nepheline does not appear without staining, but a stained section shows 

 a moderate amount of interstitial nepheline. Associated with the nepheline 

 is a little sodalite. There is no glass, and there is no cossyrite. The absence 

 of the latter is rather remarkable, considering its abundance in the Logan's 

 Point phonolite. 



Chemical Composition. — The proportions of ferric and ferrous oxides are 

 both very high, corresponding to the large amounts of . ferro-magnesian 

 minerals and magnetite present. The nepheline-syenite of Red Hill, New 

 Hampshire, U.S.A.,* has a somewhat similar composition, excepting that 

 its content of ferric oxide is much lower. An analysis of it is quoted for 

 comparison : — 



SiO^ 



Al.O., 



Fe.,0. 



FeO 



MgO 



CaO 



NaaO 



K.O 



H,0 



Ti(3., 



P.O. 



CI 



Order of Crystallisatio)!. — (1) Some 

 (2) amphibole ; (3) feldspars (after this corrosion of the feldspars and re- 

 sorption of the amphibole took place, with liberation of magnetite) ; (4) moi'e 

 magnetite and pyroxene ; (5) feldspar of the groundmass ; (6) nepheline. 



Nephelinitoid Phonolite. 



Macroscopic Characters. — The rock varies in colour from lead-grey to 

 green. It is dense, and fine-grained. Specks of dark pyroxene and large 

 phenocrysts of feldspar, as much as ^ in. in breadth, occur. The green 

 kind in particular has a platy structure. 



Microscopic Characters. — The structure is holocrystalline and porphyritic, 

 the phenocrysts, however, being rare. The groundmass in some cases ap- 

 pears cellular, and in other cases, where the nepheline is not so idiomorphic, 

 there are alternate streaks of clear, fine-grained nepheline and mossy green 

 a^girine. When phenocrysts occur they are large. 



* Baylcy, B.G.S.A., iii, p. 250. 



