390 O. E. LE KOY GEOLOGY OF RIGAUD MOUNTAIN, CANADA 



is subordinate iu amount to the feldspar, and frequent!}^ holds inclu- 

 sions of the latter mineral in a poikilitic manner. 



LAURENTIAN HORNBLENDE-GRANITE GNEISS 



The metamorphic rocks shown in the southeast corner of the map are, 

 as before stated, a hornblende-granite gneiss and amphibolite. 



The gneiss is a light colored rock of medium grain, with a rude folia- 

 tion of the essential constituents. Microscopically, it is composed of 

 allotriomorphic grains of feldspar, hornblende, and quartz, with acces- 

 sor}^ apatite, iron ore, and brown sphene. The iron ore is generally 

 surrounded by the sphene and is ))robably ilmenite. The feldspar is 

 cryptoperthite, consisting of an extremely fine parallel intergrowth of 

 orthoclase and ultramicroscopic plagioclase ; it has a specific gravit3^ ^^ 

 about 2.59, and resembles the feldspar in the orthoclase gneiss of Trem- 

 bling mountain, Quebec. The hornblende is the ordinary green variety, 

 with an extinction angle of 16 degrees. The quartz is clear, with but 

 few inclusions, and usuall}^ has an undulatory distinction. 



AMPHIBOLITE 



The amphibolite is very dark in color, rust}" in appearance, and dis- 

 integrates easily into a coarse sand. The microscope shows the typical 

 structure of a recrystallized rock, the feldspar and hornblende occurring 

 in polygonal forms arranged about a central grain. The other constitu- 

 ents are biotite, garnet, apatite, and magnetite. The feldspar is turbid 

 and is altering along the cleavage planes. It is twinned in broad lamel- 

 lae according to the albite law, and gives along the twinning plane an 

 extinction angle of 33 degrees, which indicates labradorite. This was 

 checked by a mechanical separation, which shows that it has a specific 

 gravity of 2.69. The hornblende, which is in large amount, is brownish 

 green, pleochroic, and has an extinction angle of 20 degrees. Reddish 

 brown, strongly pleochroic biotite is in smaller amount than the horn- 

 blende, but crystallized before it. Prismatic sections are lengthened in 

 the direction of the a axis and exhibit skeleton structure. The garnet 

 is fresh, pale pink in color, and perfectly isotropic. It is much traversed 

 by cracks and occurs in large, irregular individuals, either partly or 

 wholly surrounding some of the hornblende and magnetite grains. 



Relation of Rigaud to the other Igneous Hills in the Vicinity 



The examination of Rigaud has shown that both in petrographic 

 character and chemical composition the rocks are quite different from 

 those composing the other hills on the same line.. Two of these hills 



