26 N. L. Boiven — Genetic Features of 



to determine the solid phases that shall separate from a 

 liquid, the affinity of nephelite for silica being unques- 

 tionably an important factor in the present instance. 

 On account of the impossibility of distinguishing 

 between forsterite and monticellite in the small crystals 

 obtained at the relatively low temperatures, I am unable 

 to speak with certainty regarding the behavior of the 

 monticellite field. It is apparently pulled over in the 

 same manner as the other fields, indeed even in the ter- 

 nary system itself (fig. 1) it is displaced in that direc- 

 tion, so the addition of nephelite presumably emphasizes 

 this natural tendency. Monticellite may therefore sep- 

 arate from liquids not particularly rich in lime and 

 magnesia, providing they are alkalic. The manner in 

 which the monticellite field separates a part of the for- 

 sterite field from part of the akermanite field, with a 

 consequent reaction relation, has already been pointed 

 out. This separation is apparently more marked in the 

 alkalic liquids and in the case of the natural rock 

 described it appears that, for the system it represents, 

 the fields must be considered to have the relative positions 

 shown in fig. 6, which gives their projection upon the 

 Ca0-Mg0-Si0 2 plane in a manner analogous to that of 

 figs. 3, 4, and 5. Such a figure will, at any rate, afford an 

 explanation of the paragenesis of minerals in the rock 

 and is but a further modification of figs. 3, 4, and 5 in the 

 direction in which modification has been demonstrated in 

 the presence of alkali-rich liquid. The figure (6) shows 

 that the crystallization of a liquid given by the indicating 

 point may begin with the formation of olivine, which is 

 followed by pyroxene, then monticellite, then melilite, the 

 monticellite and melilite having a reaction relation to 

 pyroxene and olivine ; that is, the latter are resorbed and 

 replaced by monticellite and this in turn by melilite. 

 This is the relation that the actual minerals of the rock 

 show, with biotite as an additional replacing mineral, the 

 investigation of which would require the addition of 

 several other components. The occurrence of melilite as 

 reaction rims about augite or olivine has been noted in 

 other rocks of this area ls and indeed a great many of the 

 occurrences of melilite suggest its origin by reaction of 

 pyroxene with alkalic liquid. 



18 Robert Harvie, op. cit., p. 261. 



