54 ART. 3. — B. KOTü : 



bottom, 60 fathoms deep, was under pressure and comparatively 

 cool. The feeding channel from the main was closed by consolida- 

 tion, and the accumulation of liberated gases in the cooling secondary 

 reservoir under high tension corroded and ate away the roof, finally- 

 opening the way for the access of water to the small hot macula. 



Soon subaqueous outbursts followed, ejecting mud (fine splin- 

 ters of pumice) and lapilli in the shape of fountains, and dis- 

 turbing the body of water which caused tsunami or sea-waves to 

 the surrounding shores. The shallow bank, outlined with a 

 10 -fathom line, rose from the bottom by accumulation of the sub- 

 marine éjecta (Text- fig. 11), and likewise the lapilH-ash bed of 

 Shin-jima (No. 5), 44 m. high, was deposited and raised at this 

 time. As in normal development of submarine islands, a massive 

 dome or tholoide rose through the aqueo-pyroclastic deposit in the 

 final phase, which is typically represented in the form of Iwô-jima 

 (Nos. 3-4). 



One of the writer's motives in visiting these islands was to 

 see the products of subaqueous consolidation of lava, the modus 

 operandi of which is exactly known from authentic records. 



In contrast to subaërial lavas, compact submetallic- clinking 

 lavas were not found in the surface rocks in the whole region 

 over which the writer rowed. The rocks are all more or less pumice- 

 ous and sometimes slightly slaggy, they have the appearance of a 

 dusly coke from lignite and are also light. Under the microscope 

 the intratelluric plagioclase-phenocrysts are compound crystals witli 

 brilliant birefringence, and the crystals are full of glass due to 

 temporary regressive and corroding action of the magma during 

 crystal-growth. Hyporsthene and augite are as in terrigenous 

 lavas. In the glassy groundmass one finds only slender pyroxene 

 with stiff fibrous terminations in brown glass. The latter contains 



