Volcano ü.sliima, Idzu. 79 



and is therefore classed as " basalt ", following the definition given 

 by Iddings.'-' 



(B) Lavas of the Central Cone» 



5) Miharaite. (D. II. Fig^. 5-6. n. in. Fig?. 1-4.) 



The central cone is built up of many lava flows intercalated 

 by layers of éjecta, as has already been stated (p. 30). Though 

 each of these has certain peculiar characters, both in magascopic 

 and microscopic features, all agree so closely in their important 

 .pétrographie characters that they are rightly treated as belonging 

 to the same type. 



It is a remarkable fact that no trace of olivine has ever been 

 detected in the lavas of the central cone in contrast to the olivine- 

 bearing somma lavas. 



Older Lava (PL IL Fig. 5). — Older lavas of the central cone 

 are exposed in layers on the inner wall of the crater. The 

 specimen of which the following description is given came from 

 the lowest exposed lava on the east wall of the crater. 



Megascopically, phenocrysts of plagioclase, 2 or 3 mm. in 

 diameter, are scattered moderately, and those of hypersthene, far 

 smaller than the former, sporadically through the groundmass. 

 The groundmass is gray to black, aphanitic, and varies from com- 

 pact to more or less porous and slaggy. 



Under the microscope, calcic plagioclase and hypersthene are 

 found as phenocrysts ; and less calcic plagioclase, augite, and 

 magnetite, as the constituents of the groundmass. Interstitial glass 

 is present in variable but generali}^ in small quantities. The 

 groundmass varies withui a wide range both in granularity and 



1) lijmou-i Rocks, Vol. II., p 19r>, 1913. 



