666 HENRY S. WASHINGTON 



Cove) ; where the curve actually ascends, these would cause it 

 to descend, and vice versa. 



Three explanations may be advanced for this. The first is, 

 assuming that the covite is a primary differentiation product like 

 II, III, IV, and VI, that the course of differentiation was not 

 regular, but subject to comparatively large variations of an 

 irregular character. This seems to be unreasonable ^/r/^n, and 

 is also rendered untenable by the great regularity of the curves, 

 if this type be left out of account, by the fact that the abnor- 

 malities are systematic in direction, and correspond inversely to 

 the general characters of the respective "normal" curves, as 

 well as by the field relations of this rock. 



Another explanation is that the covite is a mixed rock, pro- 

 duced by the combination of either two differentiates of the 

 magma, or one of its differentiates with foreign rock. That it 

 cannot have been produced by mixture of arkite and foyaite is 

 clear from the fact that many of its constituents are not inter- 

 mediate between those of these two. It might have been pro- 

 duced by a mixture of foyaite and either ijolite or biotite-ijolite 

 though its position in the complex militates strongly against 

 this view. That it is not due to a mixture of the magma, or 

 parts of it, with the country rock is evident from the composi- 

 tion of the latter, which is too low in MgO and CaO to form 

 covite from foyaite, and too poor in alkalis to form it from the 

 more basic members. Its position in the mass, between the 

 foyaite and arkite, not on the extreme border, is also adverse. 



The last, and most probable, h3'pothesis is that the covite is 

 not the result of the primary differentiation which produced the 

 other types, but of a secondary differentiation of one of the 

 differentiaties of the primary process. In such a further differ- 

 entiation we would expect the same oxides to differentiate in 

 directions like those of the primary process, but in an intensified 

 degree. This, in the case of the more basic of the two comple- 

 mentary secondary differentiates, would give rise to just the 

 abnormalities noted in regard to covite. 



Of just what particular phase of the primary differentiation 



