W. G. Foye — JYephelite Syenites of Ontario. 415 



A. The Goodeeham Laccolith. 



The nephelite syenite laccolith indicated on the accompany- 

 ing sketch map by the letter A is situated in Glamorgan town- 

 ship near the village of Gooderham. It is one of a number 

 of nephelite syenite bodies lying in a syncline of limestone 

 between large areas of Laurentian gneiss. 



General Geology. 



The accompanying sketch map (fig. 1) shows the general 

 geology of the area immediately adjacent to the Gooderham 

 nephelite syenite laccolith. Limestone surrounds the body on 

 all sides. This limestone is cut by many granite pegmatite 

 dikes and has lenses of amphibolite associated with it. 



The nephelite syenite body consists of three principal rock 

 types. These are : (1) red syenite (umptekite), (2) nephelite 



Fig. 2. 



g-66g 



x<^53gg 



3 5a 1 6 75 



Fig. 2. Cross-section of the Gooderham Laccolith. 



l=Eed syenite, 2=Schistose canadite, 3=Nephelite pegmatite, 4=Amphi- 

 holite, 5= Granite pegmatite, 6=Limestone, 7=Amphibolitic limestone. 



(The figures within the sketch give the specific gravities of the several 

 rock types.) 



syenite (canadite), and (3) nephelite pegmatite. A north-south 

 cross section of the body shows the rock relationships (fig. 2). 



The specific gravities of the several rock types are indicated 

 by numbers on the section. The mass of nephelite pegmatite 

 is unduly large for the size of the laccolith. It cannot be 

 doubted that the syenite magma was surcharged with pneu- 

 matolytic gases at the time of its emplacement. The activity 

 of the gases is held to be largely responsible for the differ- 

 entiation of Ihis body. The nephelite pegmatite rests on the 

 denser canadite but the red syenite at the base has a lower den- 

 sity than the overlying canadite. At other localities the red 

 syenite cuts the canadite and again it is transitional to this 

 rock. The contact in this instance is covered by drift. 



It is quite possible that the red syenite was intruded after 

 the nephelite syenite had differentiated into nephelite pegma- 

 tite and canadite. This fact may account for the seeming lack 

 of gravitative control in the differentiation of this laccolith. 

 However, pneumatolytic transfer of the elements of the peg- 

 matite towards the roof of the body was the prime factor in 

 the differentiation. 



Am. Jour. Sci.— -Fourth Series, Vol. XL, No. 238.— October, 1915. 



27 



