PETROGRAPHY ' 883 



quite obvious that the former was directly overlain by the latter. The 

 rather abrupt slope the mountain presents along the greater part of the 

 northern border favors the conclusion that the ])resent boundary line is 

 almost correct, and approximates closely the contact between the igneous 

 and Paleozoic rocks. On the opposite side, however, the slope is very 

 gentle, and in all probability the syenite extends half a mile or more 

 farther south than the present boundary indicates. In its field relations, 

 then, Rigaud mountain is seen in contact with no other rocks. From 

 northwest to southeast it is separated from the Laurentian rocks by a 

 band of drift-covered Paleozoic strata, which varies in width from 5 to 

 13 miles. In the southeast no Paleozoic rock apparently comes between 

 the Laurentian and the syenite. It is only to the south and west that 

 the Paleozoic plain is freely and uninterruptedly developed. 



• Petrography 



hornblende syenite 



The syenite in hand specimens is a coarse grained granitic rock, com- 

 posed of feldspar and hornblende, with a small amount of gray vitreous 

 quartz. When fresh, the feldspar, which makes up the larger part of 

 the rock, is dark gray in color, with high luster and good cleavage. It 

 weathers to a red or grayish white, and is usually stained along the 

 cleavage planes by hydrated oxide of iron. The hornblende is black and 

 brilliant, showing in prismatic sections the characteristic cleavage. 

 Ordinarily the weathering processes have not affected it beyond a slight 

 deposition of limonite along the cleavage planes. Under the microscope 

 the rock presents the typical hypidiomorphic structure, and is composed 

 essentially of microperthite and hornblende, with accessory plagioclase, 

 augite, biotite, quartz, apatite, zjrcon, allanite, magnetite, and pyrite. 

 The microperthite consists of an irregular intergrowth of orthoclase and 

 plagioclase, the former greatly predominating. The orthoclase is quite 

 turbid from kaolinization, while the plagioclase, which is probably albite, 

 is usually clear and finely twinned according to the albite law, with an 

 occasional development of the pericline twinning. A few individuals are 

 twinned according to the Carlsbad law, with an irregular contact plane. 

 Many of the larger plates of feldspar include differently oriented grains 

 of hornblende and biotite, thus exhibiting the poikilitic structure. 

 Associated with the microperthite in some slides is a small amount of 

 finely striated feldspar in rounded oblong forms. It is older than the 

 microperthite, being both included in and idiomorphic toward it. The 

 maximum extinction along the twinning plane is 13 degrees, which 

 would indicate oligoclase. It is altering from the interior, the peripheral 



