New Hebrides, of 1913-14. 535 



5. The Rocks of. the 1913 Eruption. 

 The volcanic rocks of the New Hebrides, according to the previous 

 records, vary from andesites to basalts or dolerite. Professor Judd 

 (in Purey-Cust, 1896, p. 23) described the specimens collected by 

 Admiral Purey-Cust from Ambry m and some adjacent islands as 

 being all very typical augite-andesites and as strikingly uniform in 

 petrographic character. The dominant felspar he identified as 

 microtine. Sir Jethro Teall (Frederick, 1893, App., pp. 229-30), 

 who determined the collection brought by Lieut. Frederick from the 

 New Hebrides, identified the specimens from the "hot ground" at 

 50 feet above sea-level on Tongoa (one of the Shepherd Islands to 

 the south-east of Api), and from Tanna as augite andesites ; the rock 

 from the summit of Tongoa, 1,584 feet, he identified as basalt, and 

 a specimen from the island of Makura as a dolerite. According to 

 Sir Douglas Mawson (1905, pp. 459, etc.) the volcanic rocks of the 

 New Hebrides are mainly andesites and basalts, including a horn- 

 blende-andesite at Efate, and a basalt-porphyrite from Mau, an island 

 north-east of Efate. He estimates (Mawson, Proc. Linn. Soc. N.S.W., 

 1905, xxx, p. 463) the mineral composition of the latter as follows: 

 its chemical composition is given as No. 2 on p. 538. 



1st Generation. 2nd Generation. Total. 



Felspar-intermediate labradorite , 



Abi Aru . . 

 Felspar-basic andesine, Ab3 An 2 (sic) 

 Pyroxene . 

 Olivine 

 Magnetite . 



100 



Apatite . . . . minute. 



Some lavas from the 1913 eruptions collected at Dip Point, Ambrym, 

 by Professor W. M. Davis, and others obtained by Bev. P. Milne 

 have been described by Professor Marshall (1915); he identified 

 them all as basalts composed of bytownite-labradorite, diopside- 

 augite, sometimes grains of olivine, and a brown glass densely filled 

 with magnetite dust. Professor Lacroix has also described the lavas 

 from the 1913 eruption and determined them as augitic labradorites 

 too poor in olivine to be true basalts. According to his determination 

 the normative mineral composition is felspar 64 per cent, diopside 

 22 per cent, olivine 5 per cent, magnetite and ilmenite 9 per cent. 



Professor Lacroix includes an analysis by Boiteau, which is quoted 

 as No. 1 in the table on p. 538. He calls the rock an andose allied to 

 camptonose, of a kind frequent among basaltic lavas and as nearly 

 allied to those of Kilauea. Some of the old lavas thrown out by the 

 eruption he says are true basalts. 



According to Professor Iddings (1913, vol. ii, p. 648) the volcanic 

 rocks of the New Hebrides are chiefly basalts with phenocrysts of 

 olivine. 



Mr. Frater's collection includes representatives of the chief 1913 

 and 1914 lava-flows, and the rocks are all basalts, are mostly glassy, 



