[ADAMS 4 BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 37 



This represents the chemical composition of the aggregate known 

 as gieseckite, which is an alteration product of nepheline commonly found 

 in other parts of the world. Here also it evidently represents altered 

 nepheline, for although in the pink syenite the change is always com- 

 plete, no unaltered nepheline remaining in the rock, in the white 

 variety of the syenite in a few places the nepheline can be seen in pro- 

 cess of alteration into such an aggregate. A similar material is also 

 produced by the alteration of the nepheline in a nepheline syenite in 

 the township of Moniiiouth (see page 36). 



As has been mentioned, both the nepheline syenite and the reddish 

 alkali syenite frequently contain coarser-grained streaks or schlieren. 

 In this respect they resemble many of the granites of other parts of 

 this area. These coarser streaks usually coincide in direction with the 

 foliation of the rock, but in some cases they are seen to cross the folia- 

 tion of the syenite and even to penetrate the rock of the surround- 

 ing country. They are, in fact, the pegmatite phases of this nepheline 

 syenite magma. 



These pegmatites are composed of the same constituents as the 

 normal rock, and in several places on and about the Blue Mountain they 

 have been opened up by mining operations, in order to obtain the mus- 

 covite or corundum which they contain. The muscovite, which is a com- 

 mon constituent of the syenite, in some of these pegmatite dykes occurs 

 in plates several inches in diameter; while the corundum which is often 

 as has been mentioned, an accessory constituent in the syenite, in several 

 of these dykes occurs in considerable amo'unt. In these cases there must 

 have been a concentration of alumina in the residual magma represented 

 by the pegmatites in question. Those pegmatites, which are worked for 

 muscovite and corundum, however, do not as a general rule here 

 contain nepheline, or if this mineral be present, it is only a fmall amount 

 and is often represented by gieseckite, and they are thus for the most 

 part the pegmatitic developments of the alkali syenite, and usually have 

 syenite for the wall rock. 



The coarse corundum-bearing pegmatitic segregations worked for 

 this mineral on the Blue Mountain vsiry in width from one to four or 

 more feet. A striking feature presented by them is the fonn in which 

 the corundum occurs, being usually found in rounded individuals, each 

 having an irregular, though smooth, surface resembling that which 

 might have been produced by solution. Each of these rounded indivi- 

 duals is found in the middle of a large individual of muscovite, both 

 minerals being perfectly fresh and unaltered. Keference has already 

 been made to this mode of occurrence on page 22. 



