Volcauo Ôshinia, Idzu. lg 



Building Materials of the Somma. 



The best opportunity for the stractural study of the main 

 part of the somma is afforded by the exposures on the sea cUffs. 

 Those observed by tlie writer are shown in PL VI., from which 

 one can see that the somma has been built up by many repeated 

 eruptions. We can enumerate over a hundred lava flows separated 

 by the layers of éjecta — ashes, sands, lapilli, etc. Undoubtedly 

 some of those which appear to be separate flows may represent 

 merely branches of one flow, but considering that what we see 

 on the sea clifls are only those parts of many lava flows that have 

 been exposed by marine abrasion, their actual total numlier must 

 be very great. 



These lavas are of basic nature, and owing to their fluidity 

 all of them are very thin, seldom exceeding 10 m. and ranging 

 mostly from a few to several meters, sometimes even being less 

 than half a meter in thickness. 



Although all of these lavas bear close resemblance to one 

 another in their pétrographie characters, yet they may be dis- 

 tinguished into four rock-types. 



Pétrographie descriptions and intermagmatic relations will be 

 dealt with later (pp. 67 — 125), but the general characteristics of 

 each type are : — 



(1) Basaltic handaite almost free from ptienocrysts of mafic 

 minerals. 



Lavas of this type predominate and are of the widest dis- 

 tribution. 



Phenocrysts of bytownite, varying in amount, are scattered 

 through the aphanitic groundmass, gray to blaclc in colour, con- 

 sisting of labradorite, augite, mngnetite, and a smaU quantity of 



