FOYAITE-IJOLITE SERIES OF MAGNET COVE 665 



Finland and Magnet Cove in most respects, yet that here horn- 

 blende replaces augite, thus differing from other known occur- 

 rences of this rock. It seems probable that these rocks are not 

 as rich in lime as those of Magnet Cove, but higher in MgO and 

 FeO. 



In regard to the possible fourth type of laccolithic differ- 

 entiation, which was mentioned in my previous paper, namely, 

 that with a gabbroitic or peridotitic or pyroxenitic composition, 

 the suggestion may be advanced here that representatives of 

 this are to be found in the numerous sheets and dikes of diabase, 

 which, as is well known, seldom show marked differentiation 

 between the borders and the center. This is in accord with the 

 view that in these masses the "basic" solvent is present to the 

 almost total exclusion of the feldspathic portion. 



As Pirsson has already pointed out,"^ the viscosity of the 

 magma has an important bearing on the form assumed by an 

 intruded mass. The highly viscous acid magmas will tend to 

 arch up the overlying strata and form high laccoliths, while the 

 more fluid, basic magmas do not possess sufficient viscosity to 

 do this in general, and will hence form relatively thin intruded 

 sheets. But, at the same time, many of these sheets are of 

 thickness sufficient to allow of differentiation, if that had been 

 possible through the composition of the magma. 



Such sheets, then, may be regarded as the basic homologues 

 of the acid, undifferentiated laccoliths of the Mt. Henry type, 

 differing in form, but like them in that the solvent is largely in 

 excess in the magma, and hence not susceptible to differentiation. 



The abnormality of covite has been briefly noted, and a few 

 words must be devoted to it before bringing this paper to a 

 close. It will be observed on reference to Diagram 2 that for 

 this rock the positions of SiOg, Al^Og, NagO, and K^O are 

 below the corresponding abscissal points of the "normal" curves, 

 while those of (FeO), MgO, and CaO are above. In other 

 words, the positions of all the constituents of covite are consist- 

 ently inversions of what may be called the normal (for Magnet 



' L. V. Pirsson, Eighteetith Ann. Rep. U. S. Geol.Surv., Pt. Ill, p. 586, 1898. 



