
. t 
Parr Il §ii]  ROCK-FORMING MINERALS. — 71 
a Orthoclase occurs abundantly as an original constituent of many 
_ erystalline rocks (granite, syenite, felsite, gneiss, &c.), likewise in 
 eavities and veinings in which it has segregated from the surrounding 
_ mass (pegmatite). It is seldom found in unaltered sedimentary 
rocks except in fragments derived from older crystalline masses. 
It is generally associated with quartz, and often with hornblende, 
while the felspars less rich in silica more rarely accompany free 
quartz. Orthoclase is both an original constituent of plutonic 
and old yolcanie rocks (granite, felsite, &.), and a result of 
the metamorphism of sedimentary materials into foliated masses 
of gneiss and various schists. A few examples have been noticed 
where orthoclase has replaced other minerals (prehnite, analcime, 
- laumontite). 
-Orthoclase weathers on the whole with comparative rapidity, 
though durable varieties are known. ‘The alkali and some of the 
silica are removed, and the mineral passes into clay or kaolin (p. 81). 
Sanidine. Under this name is comprised the clear glassy 
_ fissured variety of orthoclase which forms so conspicuous an 
ingredient in the more silicated Tertiary and modern lavas. It has 
the same composition as orthoclase, but often with a rather higher 
percentage of soda. It occurs in some trachytes in large flat tables 
_ (hence the name “sanidine”); more commonly in fine clear or grey 
 erystals or crystalline granules, and sometimes in a vitreous con- 
_ dition (obsidian). It is an eminently volcanic mineral. In many 
lavas its large crystals are generally broken, indicative of their 
_ having already crystallized out before the lava ceased to flow; 
_ they may frequently be found full of enclosures or microliths of 
other minerals. 
Plagioclase, or Triclinie Felspars.— While the different felspars 
which crystallize in the triclinic system may be more or less easily 
' distinguished in large crystals or crystalline aggregates, they are 
difficult to separate in the minute forms in which they commonly 
occur as rock constituents. They have been grouped by petro- 
graphers under the general name Plagioclase (with oblique cleavage) 
proposed by 'schermak, who regards them as mixtures in various 
proportions of two fundamental compounds—albite or soda-felspar, 
and anorthite or lime-felspar. 
They occur mostly in well developed crystals, partly in irregular 
crystalline grains, and sometimes as a crystalline paste or base in 
which the other crystals of the rock are imbedded. On a fresh 
fracture their crystals appear as clear glassy strips, on which may 
usually be detected a fine parallel lineation or ruling, indicating a cha- 
racteristic polysynthetic twinning which never appears in orthoclase. 
A felspar striated in this manner can thus be at once pronounced to 
be a triclinic form, though the distinction is not invariably present. 
Under the microscope the fine parallel lamellation seen with 
polarized light forms one of the most distinctive features of this 
eroup of felspars. 
