110 Vol. XLIII., Art. 6. — S. Tsiiboi: 



effect (pp. 107-108). In the case of plagioclase plienocrysts, however, 

 the variation in porphyricity is too irregular to bo explained in the 

 same manner, but it may be due in some way to varying condi- 

 tions at tlie time of extrusion. 



Anorthite phenocrysts in the lava of Chigasaki enclose small 

 crystals of olivine, about (Fe,SiO,)^3_i9 (Mg.SiOJj,7_si in composition 

 (p. 90). This may be interpreted as indicating that tlie crydalU- 

 zation of the pUigloclasc began as early as in the middle of the 

 separation of the olivine crystals ichen they were still very rich in 

 forsterite. Such being the case, it is reasonable to consider that 

 there was an opportunity for the sinking of the early formed 

 anorthite crystals in company with the olivine crystals wliich 

 were separating at the same time, though the anorthite crystals 

 themselves would be very close in density to the magma, and 

 that these accunmlated toward the bottom in such a way that no 

 change of composition involving an increase of albite (the lower- 

 melting component) in crystals could be accomplished. The 

 presence of anorthite as phenocrysts in the rock of Cliigasaki may 

 be explained as a result of the processes above mentioned. The 

 bytownite phenocrysts in other rocks are considered to be, in the 

 main, of later separation from the less calcic residual liquid after 

 removal of the early formed anorthite crystals. 



The results of experimental investigations on the equilibrium 

 of various melts in the system involving the crystallization of the 

 plagioclase show that the residual liquids are always very much 

 richer in albite than the plagioclase crystals with which they are 

 in equilibriuai.'^ If, in the middle of the slow crystallization of 



1) BowEN, "TLe Melting Phenomeua of the ria-ijcl:ise Feldsparrf." Am. Jour. Sei. (4), 

 Vol, XXXV., p. 583, 1913. 



BowEN, "The Crystallization of fJaplob.isal io, FTa lodioritio and KeLttel Magmas," ihll.. 

 Vol. XL., p. ICI, 1915. 



