THE ORIGIN AND DESCENT OF ROCKS. 465 



Nosite — nosean; a complex sodium-aluminum silicate and sulphate, like 

 haiiynite, but with little calcium; common in phonolites. 



Oligoclase — a plagioclase feldspar; essentially an aluminum-calcium-sodium 

 silicate which may be regarded as a mixture of albite and a small amount of 

 anorthite; triclinic; luster vitreous, pearly, or waxy; whitish grading into greenish 

 and reddish; H. 6-7; common in crystalline rocks. 



Orthoclase — a potash feldspar; essentially a potassium-aluminum silicate; 

 varying by the replacement of the potassium by sodium and less frequently 

 by other substitutions; monoclinic; occurring in distinct crystals and also in 

 cryptocrystalline forms; cleavage planes perfect with pearly luster on cleavage 

 surface; white, gray, and flesh-red, occasionally varying to greenish white and 

 bright green; H. G-6.5; difficultly fusible; sanidine a glassy variety; felsite 

 a cryptocrystalline form; a very common mineral, especially in the granites and 

 gneisses. 



Olivine — chrysolite (q.v.)- 



Omphacite — a variety of pyroxene of grass-green color and silky to fibrous 

 luster; allied to diallage. 



Opal — silica with a varying amount of water; differs from quartz in a lack of 

 crystallization and in lower degree of hardness; amorphous, massive; some- 

 times reniform, stalac title, or tuberous; also earthy; H. 5.5-6.5; luster vitreous, 

 inclining to resinous; white, yellow, red, browTi, green, gray, blue, generally 

 pale; colors arise from admixtures; sometimes play of colors as in precious opal. 



Ozocerite — a native paraffine, mineral wax. 



Petroleum — naphtha ; a native mineral oil ; a hydrocarbon, commonly believed 

 to arise from organic matter, both animal and vegetable, but held by some to be 

 due to deep-seated chemical and thermal action. 



Pictotite — a variety of spinel, containing chromium. 



Pisolite — a concretionary variety of calcite. 



Picrolite — a variety of serpentine. 



Piedmontite — a manganese epidote. 



Plagioclase — a general term embracing the triclinic feldspars whose two 

 cleavages are oblique to each other ; embracing albite, oligoclase, andesine, labra- 

 dorite, and anorthite (q.v.). 



Plumbago — graphite (q.v.). 



Psilomelane — essentially a hydrous manganese oxide occurring in massive, 

 botryoidal, reniform, and stalactitic forms; luster submetallic; iron-black, passing 

 into dark steel-gray; H. 5-6; the common ore of manganese. 



Pseudomorph — a false form, i.e., having the form of one mineral and the 

 composition of another; usually arises from the replacement of a mineral, particle 

 by particle, by a solution of another substance, leaving the original form unchanged. 



Pyrite — iron pyrites, fool's gold, iron sulphide; isometric; commonly in 

 cubes; H. 6-6.5; luster metallic, splendent, or glistening; pale brass-yellow; 

 occurs widely disseminated throughout a large class of rocks; usually harder 

 and lighter in color than copper pjn^ites, and deeper in color than marcasite, which 

 has the same composition. 



Pyroxene — the type of a large and important group of rock-forming ferro- 

 magnesian minerals; varies in composition and embraces a large number of 



