38S PROF. P. MARSHALL OX THE SEQUENCE OF [Dec. 1914, 



II. Petrography. 



Since the series of lavas is shown so clearly that there can be no 

 doubt -whatever of the order of emission, it has been considered 

 advisable to give a brief description of the petrographical charac- 

 ters of every member of the series. In these descriptions the 

 relative quantities of the different minerals in the rocks have been 

 gauged by Eosiwahs method. 



(25) Highest phonolite. — A few irregular, partly corroded 

 crystals of anorthoclase with many minute inclusions of aegirine- 

 augite. The crystals measure 4x2 mm. Rarely idiomorphic 

 crystals of aegirine-augite measuring "8 x "15 mm. occur. 



Ground-mass : irregular anorthoclase-microlites form about 70 

 per cent, of the total, and allotriomorphic grains of aegirine-augite 

 about 20 per cent. There is a small quantity of allotriomorphic 

 nepheline. 



(24) Trachydolerite (called 'St. Leonard's Phonolite' by me 

 in 1906). — Occasional phenocrysts of hornblende, sometimes very 

 large, but usually measuring 3xl - 5 mm., and always having 

 a resorption -border of varying thickness. Strongly pleochroic ; 

 a pale yellow, /3 dark reddish-brown, y dark yellow-brown. The 

 hornblende often includes small augites. Augite-phenocrysts pale 

 pink, 3x1 mm., often with inclusions of the ground-mass and of 

 large magnetites. Slightly pleochroic. Occasionally the augite has 

 grown round the hornblende, but in such cases it has a greenish 

 tint. Both the augite and the hornblende are twinned somewhat 

 rarely. Felspar-phenocrysts are common, mainly oligoclase, but 

 there is some anorthoclase. 



Ground-mass : felspar-laths with a rough parallel arrangement, 

 •15 x '008 mm., of simply-twinned oligoclase constitute 68 per cent. ; 

 augite 32, and magnetite 5 per cent of the whole. 



(23) Basalt. — Some glomeroporphyritic patches 3 mm. in dia- 

 meter, mainly oligoclase with a little brownish-green augite. A 

 few crystals of anorthoclase, much corroded ; olivine -grains few, 

 corroded, generally stained with limonite, - 5 mm. in diameter. 

 Augite-crystals measuring 1*5 X "5 mm., idiomorphic, pale brown, 

 faintly pleochroic. In one crystal good hour-glass structure is 

 shown . 



Ground-mass : labradorite measuring "3 X "05 mm., including 

 capillary microlites of augite ; olivine infrequent, in rounded 

 grains stained with limonite; augite idiomorphic, pale green. 

 '01 x "006 mm. ; magnetite abundant. 



(22). Basalt. — Labradorite-phenociysts fairly abundant, 2 x 7 

 mm. ; brown augite, pleochroic, 5x3 mm. ; a few rounded olivines, 

 •5 mm. 



Ground-mass: indistinct flow-structure; labradorite. \15 X -03111111.. 

 57 per cent. ; pale-green augite. 02 X '008 mm.. 34 per cent. ; mag- 

 netite, - ()1 111111.. 9 per cent. 



