W. G. Foye — Nephelite Syenites of Ontario. 421 



rock is transitional to monmouthite, which was first described 

 from this locality.* 



A valley intervenes between the northern and southern por- 

 tions of the crescentic body. Masses of graphite occur within 

 this valley. They are sometimes found in the limestone but 

 more often are associated with the nephelite syenite. 



The rocks of the southern portion of the body occur in the 

 same order as those of the northern portion. A coarse-grained, 

 almost pegmatitic variety not found in the northern portion, 

 forms, however, a considerable area south of the Monck 

 Koad. 



The dips and strikes of the surrounding limestones and the 

 fact that the limestone passes beneath the northern border of 

 this body are clear indications of its laccolithic character. 



Fig. 5. 



Fig. 5. Cross-section of the Crescentic Laccolith near Tory Hill. 

 a = Garnet-pyroxene rock, b = Hornblende-nepheliterock, c= Monmouthite, 

 d=Pegmatitic nephelite syenite, gr= Limestone. 



Figure 5 is a north-south section of the laccolith. The 

 figures within the section indicate the specific gravities of the 

 rock at 20° C. These figures show that gravity controls the 

 differentiation of this mass. The rocks are all nephelite types 

 and red syenite is not associated with them. This fact 

 strengthens the opinion that the red syenite of the Gooderham 

 laccolith was intruded after the nephelite types had differen- 

 tiated, as indicated in the description of that body. 



Petrography of the Crescentic Laccolith. 



Figure 4 shows the distribution of the several rock types 

 associated with the crescentic laccolith. 



1. Pyroxene- Garnet Contact Rock. 



Macroscopic description. — The pyroxene-garnet contact rock 

 is transitional to the overlying hornblende-nepheKte rock. 

 It is reddish-black in color and is composed of greenish-black 



* F. D. Adams and A. E. Barlow, Mem. No. 6, Canada Geol. Surv., 1910, 

 pp. 274-277. 



