[ADAMS A BARLOW] ALKALI SYENITES OF EASTERN ONTARIO 63 



bite, oligoclase and andesine having been found. As in the case of the 

 nepheline syenite, separate occurrences of the rock seem to be charac- 

 terized by the presence of one variety of plagioclase to the almost com- 

 plete exclusion of another. Plagioclase often makes up from 75 to 95 

 per cent of the whole rock, the other constituents being biotite, muscovite, 

 calcite, magnetite, and occasionally corundum, scapolite and nepheline. 

 Some extreme phases of this rock are made up almost entirely of plagio- 

 clase with little or no ferromagnesian minerals. When such rocks con- 

 tain an excess of alumina which has crystallized out as corundum, they 

 are very closely' related to, if not identigal with, the rock pulmasite de- 

 scribed by Lawson.' 



The specimen selected for examination as typical of this variety of 

 rock was obtained, not from Craigmont, but from a large exposure of the 

 rock on lot 12, con. XV of the township of Dungannon. It is worked 

 for corundum at this locality, although some varieties of the corundum 

 " ore " are richer in corundum. The rock is well foliated. 



The corundum is by no means uniformly distributed through this 

 rock, and large portions are completely barren of this mineral, while cer- 

 tain rather ill-defined areas on the other hand contain a very high per- 

 centage. In outcrops exposed to the weather the corundum is very con- 

 spicuous, weathering out with pronounced relief from the surrounding 

 matrix. It occurs for the most part in small imperfect crystals and 

 grains, although occasional, characteristic barrel-shaped hexagonal crys- 

 tals are several inches in length. In freshly broken rock the corundum 

 is scarcely noticeable, unless it assumes the prevailing and characteristic 

 bluish colour. Many of the individuals have exceedingly rough and 

 jagged outlines due no doubt to the removal of much of the associated 

 micaceous material. Some of the individuals show rather perfect crys- 

 tallographic development, but, for the most part, the mineral occurs in 

 imperfect crystals or irregular grains. Much of the corundum shows 

 the peculiar parting planes or pseudo-cleavage, especially those parallel 

 to the faces of the rhombohedron and the base, both of which are per- 

 fectly developed. The colour is not uniformly distributed, but indefinite 

 patches of white, blue and brown are often noticed. Most of the mineral 

 has quite a distinct and often pronounced sapphire blue colour. Oc- 

 casional fragments show a comparatively deep brownish colour ai-mn-jerl 

 in parallel bands with well defined straight lines as boundaries. These 

 brownish streaks alternate with others, which are nearly, if not 'quite 

 colourless under the microscope. Some of the individuals have little 



' Bull. Geol. Dept. of the University of California, Vol. Ill, No. 8, pp. 

 219-229. 



