Yol. 65.] WITH POKPHTRITIC LABRADORITE-CRYSTALS. 91 



phenocrysts may be frequently seen about double tbe quantity of 

 magnetite that occurs in the rest of the ground-mass. 1 On the 

 other hand, no corresponding enrichment of the ferromagnesian 

 silicates round the phenocrysts has taken place. 



The zone that is somewhat richer in magnetite is frequeutly 

 found entirely enveloping the phenocrysts, not only on the upper 

 side but also on the lower ; thus the phenomenon cannot be 

 explained as the effect of gravity. It may be connected with 

 the relatively low temperature at the epoch of crystallization and 

 the consequently rather high viscosity of the solution. The crystals 

 of magnetite have partly been deposited on the solid bodies in the 

 liquid ; evidence of such a process is very often met with. 



The magnetite-individuals do not show an idiomorphic outline 

 against the plagioclase of the ground-mass ; hence, it may be 

 inferred that the separation of the plagioclase has continued during 

 the crystallization of the magnetite. It could not be determined 

 whether a very brief interruption of the plagioclase-crystallization 

 had taken place on account of super-saturation. 2 



At a later stage, biotite, hypersthene, and diallage began to 

 crystallize. That the separation of magnetite and plagioclase 

 still continued is showii by the non-idiomorphic boundary of these 

 two ground-mass minerals against the ferromagnesian silicates. 

 Further, I would call attention to the following point : — The 

 magnetite-individuals completely embedded or enclosed in the hyper- 

 sthene and diallage are on an average essentially smaller than the 

 magnetite-individuals not thus completely enveloped by the ferro- 

 magnesian silicates. This may be explained by the continued 

 crystallization of these latter magnetite-individuals after the ferro- 

 magnesian silicates had begun to crystallize. 



We may consequently distinguish between the following three 

 stages of crystallization : 



(1) first, plagioclase alone ; 



(2) then plagioclase and magnetite contemporaneously ; 



(3) lastly, plagioclase, magnetite, and pyroxene-minerals, besides biotite, 



contemporaneously. 



The separation of the porphyritic labradorite-crystals continued 

 until that of the magnetite began. 



The gradual change in the composition of the plagioclase and 

 hypersthene during the period of crystallization is further dealt 

 with below. 



The intimate intergrowth, with common crystallographic orien- 

 tation, of hypersthene and diallage, which has been occasionally 

 observed, may have been caused by the contemporaneous crystal- 

 lization of these two minerals. 



The inconsiderable quantity of spinel (pleonaste), estimated at 



1 See fig. 1 (p. 82) near tbe bottom on tbe right side, and other places. 



2 See my papers in Tscberin. Min. Petrogr. Mittb. vol. xxiv (1905) p. 445, 

 & vol. xxvii (1908) p. 171. 



