536 Professor J. W. Gregory — The Ambry m Eruptions, 



and usually containing some olivine. The lavas represented are as 

 follows. 



The Baulap lava pipe, according to four specimens collected 

 approximately equidistant along it, consists of olivine basalt and 

 olivine-basalt glass. 



The Craig lava-flow from the main fissures at Fo-luk discharged 

 as a lava pipe down a gully ; it then spread out as a wide sheet, 

 from which a lava pipe descended almost to the sea near Craig Point. 

 The specimens are very glassy basalts ; in one of them I observed no 

 olivine, which occurs, however, in a specimen from the lowest end of 

 this flow near Craig Point. 



The Lowea Yalley lava-flow is a lava pipe that discharged north 

 and north-west from the lava sheet, which also fed the Craig lava 

 pipe ; it is a very vesicular glassy basalt, with sparse olivine and 

 larger augites. 



Lava-flow from north of Fo-luk west-south-westward just south of 

 " 747 foot " hill. An olivine basalt with large augites. 



Harbour Crater — from the trough formed by the explosion at 

 Lon-wol-wol; a very vesicular glassy augite basalt. 



South-east of Mt. Marum, from the ventof January 1, 1914, which 

 formed the large lava sheet on the high plain to the south of 

 Mts. Marum and Benbow, and fed the Port Vato lava pipe. Glassy 

 olivine basalt with augite. 



The typical rock of the 1913 eruptions, as represented in 

 Mr. Prater's collection, is a basic lava, rich in black glass, and 

 containing glomero-porphyritic groups of a basic felspar, which 

 Mr. Tyrrell has determined as Ab 3 An 7 . The larger phenocrysts 

 show zonal structure and are often deeply corroded. The ground- 

 mass is sometimes a dense black glass, and at others consists mainly 

 of felspar laths with granules of augite and olivine. The olivine is 

 usually in small grains, often enclosed in the radial groups of felspar, 

 but it is sometimes in well-developed crystals. The proportion of 

 olivine is small and in some sections none were observed. 



Mr. Tyrrell has kindly examined the sections, and describes them 

 as follows: — 



" With the exception of slide D [from south of hill 747, west of 

 Po-luk] all the rocks consist of the same olivine-poor basalt, and 

 differ only in texture, amount of glass, and vesicularity. In general 

 the phenocrysts are comparatively sparse and small. They consist 

 of plagioclase, augite, and olivine, named in order of abundance. 

 The felspar is basic labradorite (Ab 3 An 7 ), and is developed in curious 

 little glomeroporphyritic groups with one or two crystals of olivine, 

 1 rarely with augite. The augite is a yellowish-green diopsidic variety 

 with extinction 40°. The groundmass, when dominantly crystalline, 

 consists of granular augite and magnetite, with lathy felspars 

 (AbjAnJ. The augite -f- magnetite > felspar — a basaltic type of 

 groundmass. Many of the rocks, however, contain a considerable 

 amount of dark glass and are highly vesicular. In these glassy and 

 vesicular varieties the felspar-phenocrysts appear to become more 

 abundant relative to augite and olivine; and the olivine occasionally 

 appears as tiny euhedral crystals in the groundmass. A very rough 



