[ADAMS & BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 73 



behaviour, at once removes all doubt as to the pyrogenetic origin of the 

 mineral, showing clearly, that it is one of the first products of the crystal- 

 lization of a magma supersaturated with alumina and very poor in 

 ferromagnesian minerals. The chemical analyses, as given above, are 

 in remarkably close agreement with the law formulated by Morozewicz, 

 as a result of direct experiments with artificial miigmas, not only as to 

 the conditions essential to the solution of alumina in such alumino-sili- 

 cate magmas, but also with regard to the subsequent separation of all 

 excess of alumina as corundum, as these solidify on cooling. 



SUMMARY. 



1. The nepheline and ^sociated alkali syenites of this district 

 present one of the most extensive developments of these rocks which is 

 known. They form part of the Pre-Cambrian complex of the Canadian 

 Shield, and occur along the Ijorder of the batholiths of Laurentian 

 gneissic granite, where these cut the crystalline limestones of the Gren- 

 ville series. 



2. They differ from most occurrences of these rocks which have 

 been described from other parts of the world, in that they have not 

 usually the massive character of ordinary intrusives, but possess a dis- 

 tinct gneissic structure. 



The gneissic or foliated structure which they usually display is 

 combined with a schlieren or streaked structure, which gives rise in 

 small exposures, to a banded appearance, the several bands dif- 

 fering in the relative proportion of constituents present. The foliation 

 is not such as would be produced by the direct crushing of a massive 

 rock. Cataclastic structures are very seldom seen, and the rock very 

 rarely shows any distinct evidence of pressure. The arrangement of the 

 component minerals, with their longer axes in the same direction, pro- 

 duces the foliation, while their variation in amount from band to band 

 serves to emphasize it. The rock is,. as a general rule, poor in iron mag- 

 nesia constituents, and its appearance on the weathered surface so closely 

 resembles that of the crystalline limestones of the Laurentian, which 

 are often more or less impure from the development of secondary sili- 

 cates in streaks and bands through them, that it is often impossible to 

 tell the two rocks apart at a distance of a few yards. 



3. The nepheline syenite magma throughout the area is one which 

 was relatively very rich in soda. The plagioclase present is in most 

 cases aJbite, but sometimes oligoclase or even andesine, and these felds- 

 pars preponderate largely over the orthoclase and are frequently the only 

 feldspars which the rock contains. The rock furthermore differs from 



