ROCKS 



142 AKT. 2. — B. KOTO: 



new crater-cone on the maps at my disposal. But no one can fail 

 to recognize in this report a record of a distinct volcanic action. 



easalt The island is built up solely of basalt of which there are 



in my possession only a few specimens of gravel collected near 

 the island capital by a member of the staff of the Moh-pho 

 Consulate, and one chip from the south coast given me by Mr. 



KOBAYASHI. 



My handspecimens are either slaggy, even vesicular, or com- 

 pact, and all are of a grayish color. 



a) Macroscopicallj visible phenoerysts are plagioclase. The structure 

 presents various degrees of coarseness, some portions having 

 the phenocryst of plagioclase 8 mm in length. Prevailing rocks 

 have compact structure. Microscopically, the coarse variety 

 consists of lath-shaped bytownite (equal extinction-angle being 

 30°) with the usual albite-twinning, and crystals and grains of 

 olivine rich in iron, both being abundantly present. Tlie rest 

 of the mass is made up of brOAvnish-yellow grains and brownish 

 globulite in a colorless liase. With weak powers the ground- 

 mass appears entirely black. 



6 The compact variety appears bhiish-black with no phenoerysts. 

 Microscopically, only plagioclase crystals are seen in the dark 

 globulitic granular groundmass. Olivine and augite are not 

 found. 



So far as I can judge from examination of my handspecimens, 

 the prevailing rocks of the island are rich in plagioclase and 

 olivine with no augite-phenocrysts in the blackish groundmass of 

 the globulitic base mixed with grains of augite. It is the Korean 

 type of the plagloclase-hasalt . which is extensively developed in 

 North Korea, forming basalt- mesas. 



