J44 "^'ol. XLIII, Art. 6. — S. Tsiiboi: 



is summarized as follows : — 



The body which now constitutes the somma is the earliest of 

 those so far seen. It was very gradually built up l>y repeated 

 volcanic actions without any prolonged intervals of quietude. 

 During the development of the somma, but far before its skirt 

 reached the present state of development, extruded lavas com- 

 paratively rich in olivine formed small igneous bodies in the north- 



A 



western part of the present Oshima, isolated at that time from the 

 main body by the sea. These soon expired and were destroyed 

 by the marine erosion. When the development of the somma was 

 nearly completed, the steep slope running in the direction N.-S. 

 along the east coast of the island was formed by the depression 

 of the ground. Vulcanism displayed its energy not only at the 

 centre of the volcano but also on the flanks of the somma at 

 several spots during various but comparatively later stages in the 

 volcanic history. The products of these flank eruptions are es- 

 pecially abundant in the southeastern part of the island. The 

 depression of the summit of the somma resulted in the formation 

 of a huge caldera whereupon the encircling wall w^as badly broken 

 in its northeastern quarter. The central homate appeared in the 

 caldera and its products not only filled the inside of the caldera 

 but spread dowai to the sea shore, especially abundantly in the 

 east through the northeastern gap of the ring-wall. In historic 

 times the volcano continued frequently to display its activity, and 

 eruptions in the following years are on record: 084, 1112, 1416, 

 1421, 1600-1001, 1612-1013, 1636-1037, 1084-10l»0, 1777-1778, 

 1803, 1822, 1840, 1870, 1870-1877, 1912-1914, and 1915. 



(5) Turning to the problem of the magma, the original magma 

 is supposed to be basaltic, highly fluidal and with a specific gravity 

 nearly equal to that of the calcic plagioclase crystals. The magma 



