214 "^'ol. XLIIL, Art. fi. — S. Tsuboi : 



•of augite were to proceed further. 



Since the specific gravity of the pyroxene crystals is believed 

 to be much higher than that of the magma from which they were 

 precipitated (p. 109) and the cooling of the magma to have been 

 very slow (p. 108), a sufficient opportunity must be allowed for the 

 sinking of the pyroxene crystals. Pyroxene phenocrysts are very 

 scarce in most of our rocks. This fact was also noticed by Yama- 

 SAKi^^ who regarded it as one of the characteristic features of the 



A 



rocks of Oshima. This scarcity of pyroxene phenocrysts is under- 

 stood when it is considered that most of the Oshima rocks represent 

 the ujDper portion of the magma basin from which the lieavy 

 pyroxene crystals had been removed by their subsidence eir(.'cted 

 by gravitation. 



Magnetite. — Magnetite is, f(3r the most part, a product of the 

 period after tlie magma was extruded as lavas. The probability 

 of this iiiferenco is supported by: — (1) Magnetite occurs only as 

 very small grains constitutiii.r tlio groundraass and never as phono- 

 erysfs ov as inclusions in other phenocî -yàtic minerals. (2) The 

 form of magnetite varies witli the crystallinity of the ground- 

 mass ; tlms, the glassy groundmass of some rocks (for example, 

 the hyalocrystalline part of miharaite, p. 81) is almost free from 

 visible magnetite crystals ; in the groundmass with a little higher 

 crystallinity, it disseminatus in very fine, often skeletal crystals ; 

 while it is in euhedral crystals in the groundmass of high crys- 

 tallinity. If the magnetite crystals belonged chiefly to the intratel- 

 luric separation, they would be found in euhedral forms in every 

 rock, whatever the crystallinity of its groundmass may be. 



Groundmass. — The groundmass is the product of sudden cooling 



1) "Report on the Volcano Oshima," Report Eiik/. Inoetsl. Gm., No. 9, p. 50, 1893 (i:i 

 Japanese). 



