.()2 Vol. XLIIL, Art. 6. — S. Tsnbai : 



desites." Iddings's term " bandaite,'"^ which includes the quartzose 

 ajjhanites characterized by normative labradorite, is not only sug- 

 gestive of the intimate relation of our rocks with " pyroxenc- 

 andesites " but also clearly implies the oversaturation of silica with 

 respect to other components. 



The application of the term " basalt "'^ in this paper is restricted 

 onl}^ to those rocks so rich in olivine that they are inferred to 

 belong to the j^erfelic order. 



To the rocks of the central cone, a new name " miharaite " 

 has been givon on account of their peculiar chemical compositions 

 which will be given later (pp. 87-88). 



Some Specially Devised Pétrographie Methods. 



For the exact identification of the rock-forming minerals, the 

 w^riter wished to determine their optical properties as far as 

 circumstances permitted. But, since few of the methods of optical 

 measurement have proved to be applicable owing to the minute- 

 ness of the mineral grains, the writer has introduced some specially 

 devised ones, of which brief notes are given in tho following : — 



Refractive Indices. — The refractive indices ol the minerals 

 were determined by the BECKE-line method, immersing the mineral 

 grains in liquids of the known indices. With a biaxial minenil, 

 observations were made twice on each grain above a nicol, placing 

 iirst one of the vibration dh-ectious of the mineral grain and then 

 the other parallel to that of the nicol. Repeating the process on 

 a large number of grains of various orientation with different 

 liquids, the highest and the lowest values of the refractive indices, jr* 

 and a', were obtained, and then r and '/ were expressed as fol- 



1) Igneous Rocks, Vol. II., p. Ill, 1J13. 

 2, Ibid., ])p. li)G-19J. 



