pe a 
Etta | GEOGNOSY. 

The following table shows the average composition of the chief 
triclinic felspars. 
















oe Alu- - Hard- Spec. aya 
Silica.| Jina. Potash.| Soda. | Lime. | “jocg. Gravity. Habitat, 
SF Microcline | 64°30 | 19°70 | 15°60 | 0°48 “6 6°0 2°540 |In some syenites, &c. , 
ee | , 
ZA) arpite . . | 6362) 19°56| .. | 11:82] .. | 66-5 | 2°59—264 |In some granites, and in 
as several volcanic rocks. 
-/Oli 63°70 | 23°95 | 1°20] 8°11 | 2°05 6°0 | 2°60—2°66 In many granites and 
sates Oligoclase other eruptive rocks. 
EE 
a! Andesine - | 63°85 | 24°05 | 0.88 | 5.04 5°04 | 5—6 | 2°66—2°69 \In some syenites, &c. 
"5 \Labradorite} 52°90 | 30°30 ie 4°50 | 12°30 6'0 | 2°68—2°74 |Essential constituent of 
many lavas, &c., abun- 
dant in masses in azoic 
bite : rocks of Canada, &c. 
2 =) Anorthite- | 43°08 | 36°82 ae hie 20°10 6'0 | 2°67—2°76 \In many volcanic rocks, 
Ae sometimes in granites 
12 and metamorphic 
rocks. 



eR TC A rE aE ETE 
The triclinic felspars have been produced sometimes directly 
from igneous fusion. This can be studied in many lavas, where one 
of the first minerals to appear in the devitrification of the original 
molten glass is the labradorite or other plagioclase. In other cases 
these minerals have resulted from the operation of the processes to 
which the formation of the crystalline schists was due; large beds 
as well as abundant diffused strings, veinings, and crystals of triclinic 
felspar (labradorite) form a marked feature among the ancient 
eneisses of Hastern Canada. The more highly silicated species 
(albite, oligoclase) occur with orthoclase as essential constituents 
of many granites and other plutonic rocks. The more basic forms 
(labradorite, anorthite) are generally absent where free silica is 
present; but occur in the more basic igneous rocks (basalts, &.). 
Considerable differences are presented by the triclinic felspars in 
regard to weathering. On an exposed face of rock they lose their 
glassy lustre and become white and opaque. This change, as in — 
orthoclase, arises from loss of bases and silica and from hydration. 
Traces of carbonates may often be observed in weathered crystals, — 
The original steam cavities of old volcanic rocks have generally been 
filled with infiltrated minerals, which in many cases have resulted 
from the weathering and decomposition of the triclinic felspars. 
Calcite, prehnite, and the family of zeolites have been abundantly pro- 
duced in this way. ‘The student will usually observe that where these 
minerals abound in the cells and crevices of a rock, the rock itself ig 
for the most part proportionately decomposed, showing the relation 
that subsists between these infiltration products and the decomposi- 
tion of the surrounding mass. Abundance of calcite in veins and 


