ß3 Vol. XLIII., Art. 6. — S. Tsuboi : 



is meagrencss of phenocrysts of mafic minerals. 



3Iof1e of Occurrence. — The somma lavas of this t5^pe are of 

 the widest distribution. They are found through all parts of the 

 island in numerous flows, both superfluent and effluent, alternated 

 by layers of clasmatic materials and constitute the main part of 

 the somma. The best exposures of these lavas are met with on 

 the precipitous cliffs that surround the island (PL YI.) and on the 

 ring-wall at the top of the somma. 



3Iegascbpic Characters. — The rock varies in megascopic char- 

 acters from only slightly to strongly porphyritic, plagioclase con- 

 stituting the most conspicuous phenocrysts. They are usually 

 equant in development, very often grouped in cumuloporpbyric 

 fabric, and vary in diameter from 0.2 to 3 mm. In most lavas, 

 small sporadic phenocrysts of olivine are found, but in some, they 

 are wholly wanting. In rare instances, it happens that small 

 phenocrysts of hypersthene and augite occur in negligible amount. 

 These phenocrysts are scattered through the groundmass with no 

 trace of any regular arrangement. - 



The groundmass is megascopicaliy of two types : («) black in 

 colour and generally porous and slaggy, and {fi) gray in colour 

 and rather compact or almost free from pores and vesicles ; the 

 two, however, are connected by intermediate types. The differences 

 between these types are attributable merely to the varying 

 conditions under which the rocks consolidated. As a rule, the 

 surficial part of each lava has the groundmass of «-type an<l tlie 

 iîmor part that of ;5-type. 



Microscopic Characters, — Under the microscope, phenocrysts 

 of plagioclase are always seen, but those of other minerals rarely 

 appear in thin section. 



Phenocrystic plagioclase is generally euhedral to subhedraU 



