Volcano Ôshima, Idzu. 45^ 



had destroyed these igneous bodies that these islets were connected 

 with the main body. 



As to their probable birth-time, we have no direct evidence, 

 but that such small igneous bodies as these still maintain their 

 existence, though now in a highly demolished condition, in spite 

 of the constant action of the sea, suggests that their birth was- 

 not at a very remote date. It would seem to be most reasonable 

 to assume that they wore born in the middle of the volcanic 

 history of the main cone, soon after which they ceased to appear. 



It is noticeable that the lavas constituting these igneous bodies 

 are all comparatively rich in olivine and pyroxene, in contrast to 

 the common somma lava in which these minerals are either absent 

 or found only in a negligible amount. 



Since these olivine-rich lavas are considered to have been 

 discharged from the local vents while the main centre was pouring 

 out olivine-poor lavas, the peculiar character of the former can not 

 be attributed to w-hat is called the magmatic cycle. 



It appears likely that these ohvine-rich lavas constituting the 

 small igneous bodies were derived from the lower part of a local 

 satellitic magma reservoir. On this supposition all the pétrographie 

 facts, especially the occurrence of olivine which the writer believes 

 to be due to gravitational control, seem to be best explained. This 

 point will be discussed fully on pp. 118 — 119. - 



The view, that the la^as of the demolished igneous bodies 

 came from a local satellitic magma reservoir, explains the short 

 life of the vents of these bodies. As to the persistence of vents 

 in general, it is believed to be due to the action of "gas- 

 fluxing " or " blow -piping "'^ ; so if the vents now in question are 



1) Dlay, "The Nature of Volcanic Action," Fror. Am. Acad, of Art.s and Sciences, VoU 

 XLVII., I'Jll. 



