Mishima Island is composed mostly of the basaltic rocks of the alkali rock series and partly of the tholeiitic basalt, which were erupted in the Miocene or Pliocene to Pleistocene. Eleven lavas of different periods which are chemically analysed, are classified into the tholeiitic basalt, picrite-basalt, trachybasalt and trachyandesite. These rocks are generally rich in MgO, and particularly its content is as high as 14.2% in the picrite-basalt. This is the first occurrence of picrite-basalt in Japan. It is a distinct character of the rocks of this island that they devoid entirely hypersthene either as phenocryst of groundmass, though they contain abundant xenocrysts and xenolithes. Thus these rocks belong not to the calc-alkali rock series, but to the alkali rock series. This is a remarkable diference from the rocks of Iki Island, Where many calc-alkalirocks are derived from the contamination of the alkali rock series, as pointed out by Dr. Aoki. This fact is interesting from the genetic consideration on the magmatic differentiatin in this island, and will be discussed later.