104 PETROLOGY OF THE ALKALINE ROCKS 



The pyroxene phenocrysts vary from true segirine in the most acid and alkaline 

 rocks of the series to a brown titaniferous pyroxene in the most basic. Both the 

 felspars and pyroxenes just mentioned occur also as constituents of the base and 

 smaller phenocrysts of a second generation. 



Olivine occurs as corroded phenocrysts, very rarely as a constituent of the base. 



Magnetite belongs partly to the first and partly to the second generation. Other 

 iron ores (haematite, limonite, ferrite, opacite) occur chiefly as secondary products. 

 Ilmenite frequently takes the place of magnetite. 



Leucite occurs in some of these rocks as idiomorphic phenocrysts, rarely of the 

 first, but usually of the second period of crystallisation. It is rarely quite fresh. 

 Commonly it is replaced more or less by zeolites and pseudoleucite (orthoclase and 

 nepheline). 



Other felspathoids, chiefly nosean, sodalite, and analcite, occur occasionally in the 

 interstices of the base. 



Apatite occurs both as small first generation phenocrysts, and as a constituent of 

 the base. 



The other minerals recorded and described under the Trachytes are also present in 

 variable amount, especially the soda amphiboles. 



Texture. — The structure of these rocks is usually vesicular and always porphyritic, 

 namely, porphyritic hiatal, megaphyric, and usually they are dosemic, though 

 dopatic types do occur. The crystallinity varies from holocrystalline to 

 hemivitreous. The base may be anything from glassy to holocrystalline with 

 phenocrysts ranging from microporphyritic to magniphyric in size. The fabric of the 

 base may be trachytic, pilotaxitic, hialopilitic, or vitrophyric. 



Inclusions. — Inclusions of a white rock determined by Mr. J. Allan Thomson to be 

 sanidinites and microtinites occur in some kenytes. In others (kenytic breccias on 

 Parasitic Cone of Erebus) we have inclusions of an oligoclase trachyte. The more 

 basic kenytes have frequently inclusions of the more acid types. 



Order of Consolidation of the Minerals. — The minerals belonging to the intra telluric 

 period of crystallisation crystallised in the following order : 



(a) Orthoclase in rounded fragments included in the anorthoclase phenocrysts. 

 This may be xenogenic in origin, the remains of partially resorbed country rock. 



(b) Magnetite as grains included in phenocrysts of other minerals ; and also as 

 independent grains. 



(c) Pyroxene, both as independent crystals and inclusions in felspar. 



(d) Anorthoclase as corroded almond-shaped phenocrysts. 



(e) Leucite occurs occasionally as a first-generation mineral in the more basic facies 

 of kenyte. 



All these minerals belonging to the first generation exhibit great corrosion by the 

 magma, and in the case of the felspars strain phenomena. 



The minerals of the second generation crystallised with few exceptions in the 

 following order : (a) magnetite ; (b) olivine ; (c) apatite ; (d) titaniferous pyroxene 

 and tegii'ine-augite ; (e) felspar ; (/) aegirite, glass, and felspathoid. 



Habit of Felspar. — The felspar of the kenytes consists of three kinds : 



(1) Almond-shaped corroded anorthoclase crystals, grading into microcline 

 microperthite, and moiree niicrocline; 



(2) The idiomorphic prismatic and tabular soda sanidine or anorthoclase of the 

 base; and 



(3) The tabular corroded phenocrysts of acid plagioclase in many of the basic 

 kenytes. 



