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bei-ed conical shell, like a straight horn. Elym., opfof, orthos, 

 straight, and xepus, ccras, horn. 



Outliers. When a portion of a stratum occurs at some distance, de- 

 tached from the general mass of the formation to which it be- 

 longs, some practical mineral surveyors call it an outlier, and 

 the term is adopted in geological language. 



Oxide. The combination of a metal with oxygen ; rust is oxide 

 of iron. 



Oxygen. One of the constituent parts of the air of the atmosphere; 

 that part which supports life. For a further explanation of the 

 word, consult elementary works on chemistry. 



Pelagian, Pelagic. Belonging to the deep sea. Etym., pelagus, 

 sea. 



Petroleum. A liquid mineral pitch, so called because it is seen to 

 ooze like oil out of the rock. Etym., petra, rock, and oleum, 

 oil. 



Pisolite. A stone possessing a structure like an agglutination of 

 peas. Etym., mtfov, pison, pea, and Xidoj, lithos, stone. 



Pit Coal. Ordinary coal ; called so because it is obtained by sink- 

 ing pits in the ground. 



Pitch Stone. A rock of a uniform texture, belonging to the unstra- 

 tified and volcanic classes, which has an unctuous appearance 

 like indurated pitch. 



Pliocene. This is derived from ^Xsiwv, more, and xawoj, recent, and is 

 founded upon the existence, in the beds which it embraces, of 

 a greater number of recent than extinct species. The pliocene 

 rocks are referred to two periods, the Older Pliocene and Newer 

 Pliocene ; In the newer pliocene, the number of extinct species 

 is extremely small. 



Plutonic Rocks. Granite, porphyry, and other igneous rocks, sup- 

 posed to have consolidated from a melted state at a great depth 

 from the surface. 



Porphyry. An unstratified or igneous rock. The term is as old as 

 the time of Pliny, and was applied to a red rock with small an- 

 gular white bodies diffused through it, which are crystallized 

 felspar, brought from Egypt. The term is hence applied to 

 every species of unstratified rock in which detached crystals of 

 felspar are diffused through a base of other mineral composition 

 Etym., Ttopep-jpa, porphyra, purple. 



