RELATIONSHIP TO OTHER HILLS ^ B91 



have recently been worked up, and it has been found that the prevailing 

 syenite, which is an alkali rich one, is associated with a more basic rock, 

 such as theralite or essexite. Unless, therefore, the character of the 

 magma changed considerably, Rigaud could hardly be regarded as being 

 genetically connected to the rest of the series. On the other hand, con- 

 sidering the fact that Rigaud is associated with the older crystalline 

 rocks, it seemed advisable to visit the nearest eruptive in the Laurentian 

 to ascertain if it would have any relations in common with Rigaud- 

 Accordingly a brief examination was made of an area of hornblende 

 syenite which occurs on the border of the Laurentian in the townships of 

 Grenville, Chatham, and Wentworth, about 13 miles northwest of Rigaud. 



Grenville Area of Syenite and Porphyry 



The Grenville syenite occupies an area of 36 square miles, and was 

 described by Sir William Logan.* It cuts the Laurentian rocks on 

 three sides, but to the south it is overlain unconformably by the Upper 

 Cambrian. Like Rigaud, it is pierced in the western part by a small 

 pear-shaped mass of porphyry. 



Petrography 



hornblende syenite 



Megascopically, the syenite bears a close resemblance to that of Rigaud, 

 but contains a larger proportion of quartz, which increases in amount 

 until finally the rock passes into a granite. Microscopically, it is com- 

 posed of feldspar and hornblende, with accessory augite, brown biotite, 

 quartz, apatite, zircon, and iron ore. The feldspar varies in different 

 Parts of the area from an unstriated one to a microperthite, both being 

 much kaolinized. The unstriated feldspar is homogeneous, and occurs 

 in thick lath-shaped and irregular individuals, while the microperthite 

 consists of a parallel intergrowth of the orthoclase and a finely twinned 

 plagioclase. The latter has a better form than the orthoclase, and from 

 its association is older. It occasionally occurs as Carlsbad twins, and is 

 sometimes intergrown with hornblende in a graphic manner. The 

 cleavage of both varieties is not ver}^ distinct. 



The hornblende is in comparatively small amount, the larger grains 

 being quite irregular, but many of the smaller, when they are included 

 in the feldspar, are idiomorphic. The pleochroism varies from deep 

 green to pale yellow, and the extinction angle is 19 degrees. The strong 

 allotriomorphism toward the feldspar is not so common as in the Rigaud 

 syenite. Some individuals are intergrown with a pale yellow augite, in 



*Geo;ogj^ of Canada, 1863 Report, p. 40. 



