168 THE CANADIAN NATURALIST. [June, 



grains of feldspar were separated from the heavier minerals, and 

 found to have a specific gravity of 2.561. The result of its anal- 

 ysis, which scarcely differs from that of Brome, is given under iv^ 



II. III. IV. 



Silica 65.70 65.30 65.15 



Alumina 20.80 20.70 20.55 



Lime 84 .84 .73 



Potash 6.43 .... 6.39 



Soda 6-52 6.67 



Volatile 50 .... .50 



100.79 99.99 



Yamaska Mountain. — About twelve miles to the north of 

 west from Shefford Mountain rises the hill of intrusive rock known 

 as Yamaska Mountain, which has an area of about four square 

 miles, and breaks through the strata of the Quebec group, near the 

 line of the great dislocation which brings these up against the 

 limestones of the Trenton group. The southeastern part of this 

 hill consists of a granitoid diorite hereafter to be noticed ; but the 

 greater portion of the mass may be described as a granitoid tra- 

 chyte, differing in aspect from that of Brome and Shefford, in 

 being somewhat more micaceous and more fissile. The mica, which 

 is dark brown, is in elongated flakes, and there is neither horn- 

 blende nor quartz in the specimens collected, which however hold 

 small portions of magnetite, and minute crystals of amber-yellow 

 sphene. These seem to be contained in veins of segregation, which 

 are of a lighter color than the mass. The cleavable feldspar 

 grains, which make up by far the greater part of the rock, are 

 brilliant, with a vitreous lustre, and are often yellowish or reddish- 

 gray in color. A portion of this feldspar separated by washing from 

 the crushed mass of the rock, had a specific gravity of 2-563, and 

 gave by analysis the result v. Another portion of selected grains 

 of the feldspar gave vi. Both specimens were however somewhat 

 impure. 



V. VI. 



Silica 61.10 58.60 



Alumina 20.10 21.60 



Peroxyd of iron 2.90 2.88 



Lime 3.65 5.40 



Magnesia 79 1.84 



Potash 3.54 3.08 



Soda 5.93 5.51 



Volatile 40 .80 



98.41 99.71 



