[ADAMS à BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 23 



has the power to dissolve alumina and on cooling to separate out any- 

 excess completely. The conditions laid down by what is known as Moro- 

 zewicz's law (see page 65) are completely fulfilled by the corundum- 

 bearing nepheline syenites of Ontario. In all magmas, those of acidic 

 composition especially, Avater is believed to be present in considerable 

 amount. As the corundum separated out, the magma would tend to ap- 

 proach more nearly to the composition of a mass of fused feldspar to- 

 gether with a certain amount of water. At this stage, and on account of 

 some condition or change of conditions, this residual magma attacked 

 the corundum, the hydrous feldspathetic magma together with the 

 alumina fl'om the dissolved corundum making muscovite, which crystal- 

 lized around or replaced the corundum, according to the following for- 

 mula : — 



K2O, AI2O3, 6SiO, (= orthoclase) + 2AUO3 (= corundum) + 2H.0 

 = K,0, 2H2O, 3ALO3, eSiO, = muscovite. 



This likewise explains the marked prevalence of this altera- 

 'tion in the pegmatitic fades of the syenite, for it is in these residual 

 differentiated portions of the magma that water plays such an important 

 part in the process of crystallization. 



Corundum. — The crystals when normally developed are usually six- 

 sided prisms which are sometimes terminated by a six-sided pyramid 

 and not frequently by the basal plane. Many of the ciystals especially 

 those occurring in the nepheline syenite have a tolerably sharp and 

 perfect outline, frequently tapering to either extremity, thus producing 

 the very characteristic barrel-shaped form. The pyramidal and pris- 

 matic faces are very often more or less deeply striated or grooved horizon- 

 tally. The basal planes or truncated ends of the crystals are frequently 

 striated in three directions, forming equilateral triangles, corresponding 

 with the less perfect rhombohcdral partings or pseudo-cleavages. The 

 crystals vary greatly in size, the largest noticed in the nepheline syenite 

 being about eight inches in length by two inches in diameter. Such 

 crystals are comparatively rare, the usual size being about two to three 

 inches and from that sinking to those of microscopic dimensions. The 

 larger crystals as well as the very small ones are usually inclined to 

 have an irregular or imperfect outline. The conmdum is in many in- 

 stances somewhat l^rittle, breaking with an uneven or eonchoidal fracture, 

 but when in large masses it is exceedingly tough. The lustre is in 

 general vitreous, hut in the translucent light green variety noticed in 

 Brudenell township the lustre is somewhat pearly. The colour of the 

 corundum in the nepheline syenite is in general of varying shades of 

 blue to white. It is' sometimes of a distinct rose-red colour. Many of 

 the crystals, especially those present in the nepheline syenite exposiires 

 in the vicinitv of York river, show an irregular or cloud-like arrangement 



