Marshall. — Volcanic Eruptions on Ambrym Island. 391 



brought from this island. Some of these have been kindly forwarded by 

 the Eev. Peter Milne, a missionary on Nguna Island, and others by Professor 

 W. Morris Davis. 



The locality of the specimens sent by Mr. Milne is not stated, but Dr. 

 Bowie assures me that the rock is the commonest type that is found at the 

 western end of the island. This rock is highly vesicular, and contains large 

 rounded phenocrysts of feldspar as much as 1 cm. in diameter, but no other 

 minerals can be distinguished in hand-specimens. 



In section the rounded crystals of feldspar are found to be aggregates 

 of bytownite, with an extincton-angle of 40° in those sections which are 

 at right angles to the brae hyp inacoid. In the central portions of these 

 aggregates there are a great many inclusions of magnetite and of glassy 

 matter. These can be compared with the feldspar glomerules of the Mau 

 basalt previously described by Mawson.* In the present instance, how- 

 ever, there is not a marginal zone of inclusions, nor are the marginal portions 

 less basic than the central area. Smaller crystals of greenish augite and of 

 perfectly colourless olivine also occur, and in some instances rounded grains 

 of olivine are actually embedded in the feldspar. The groundmass consists 

 almost equally of microlites of bytownite, short crystals but more frequently 

 rounded grains of augite, and sometimes also of olivine. There is a con- 

 siderable residue of brownish glass, densely filled with magnetite-dust. 



From Dip Point Professor W. M. Davis has sent me a specimen of a 

 compact grey rock which contains numerous feldspar phenocrysts. As in 

 the previous case, the phenocrysts when examined with the microscope are 

 found to be aggregates of several crystals, though they are less regularly 

 arranged than in the preceding rock. The mineral in these aggregates is 

 again bytownite. The groundmass consists mainly of bytownite with large 

 grains of diopside and of colourless olivine, and small octahedrons of 

 magnetite. There is a base of brownish glass, which contains numerous 

 needles of ilmenite. 



A specimen of the lava of 1913 at Dip Point was also sent to me by 

 Professor W. M. Davis. The hand-specimen is iron-grey, and it is highly 

 vesicular. There is no porphyritic feldspar, though green augite and olivine 

 are distinct. In section there is found to be much feldspar in crystals of 

 small size but much twinned ; it is of a basic type, bytownite-Iabradorite. 

 The augite is near diopside, but slightly pleochroic. The crystals of olivine 

 are generally crowded Avith magnetite-dust. This appears to be an alter- 

 ation of a somewhat unusual nature, as no mention of it can be found 

 in text-books. A similar alteration occurs in the olivine of a dyke which 

 traverses the scoria cone of Mount Eden, at Auckland, and also in basaltic 

 rocks which occur near the margin of the pipe of the great central volcano of 

 Tahiti, though in the last instance the change is marginal or restricted to the 

 border of the cracks which traverse the mineral. It is still more noticeable 

 in the peridotite that forms part of the pipe of Tahiti. 



The groundmass of this Ambrym rock consists of feldspar and augite 

 with magnetite. The augite of the groundmass is greenish, but often with 

 a brown margin. 



It will be seen that these rocks are all basalts, and it is stated by Mawson 

 that all the recent products of volcanic activity in the New Hebrides are 

 basaltic, though earlier eruptions were associated with the emission of 

 andesitic rocks. 



* "Geology of New Hebrides," Proc. Linn. Soc. N.S.W., 1905, p. 463, pi. xxiii, fig. 4. 



