22 KOTO : NOTES ON THE GEOLOGY 



in sj^eaking of bastite, says ' die Umbildung scheint in 

 hohem Grade durch die gleichzeitige Anwesenheit des Olivin 

 und dessen Umwandlung zu Serpentine befördert zu werden.' 

 Chemically speaking, there exists a close resemblance between 

 iddingsite and the * crystallised diallage ' of Baste^', considering 

 out of question a trace of alumina. Optical schemes differ, of 

 course, in the two minerals, but I could not make out surely 

 the optical orientation of iddingsite in my slides, on account 

 of its extremely line lamellar structure. 



PLAGIOCLASE. 



Plagioclase has, generally speaking, crystallised out in a 

 single generation of the flow period. Diflering from the Ande- 

 sitic plagioclases which present various dimensions, the felspar 

 of Basalt is uniform in size. It is, however, not wanting in 

 large, phenocrystic crystals in some slides, which also belong 

 to the products of the effusive period, slightly earlier in crys- 

 tallisation than the ones in the general mass ; for, the small 

 laths of plagioclase ai-e partly embraced by the phenocr3'sts, — 

 a fact which also leads me suppose that the plagioclases have 

 grown in a comparatively motionless magma. They show no 

 signs of corrosion, so common in the olivine of the intratelluric 

 origin, though the effects of tossing and fracturing of crystals 

 are by no means seldom observed. 



The phenocrystic plagioclase {PI. II, Fig. 2) has a tabular 

 form on M, somewhat elongated towards the vertical axis. Zonal 

 structure is rare in contrast to the Andesitic felspar ; the same 



1) Hintze, 'Handbuch dei- Mineralogie,' Bd. U., S. 972. 



