444 



Pleoehroism The property possessed by pellucid doubly refractive minerals of 

 transmitting rays of different colour or intensity in different directions. 

 Uniaxial minerals are dichroic, since they possess two directions (parallel 

 and normal to the chief axis) in v.hich the greatest difference of colour is 

 shown. Biaxial minerals, on the other hand, are trichroic, since they 

 have three directions, corresponding to the three different axes of elasticity, 

 along which rays of different intensity are transmitted. Syn. Polychroism. 



Plexus A network, 



Plusiatic A name sometimes applied to sands which are rich in ore or precious 

 stones. 



Plutonic In petrography, applied to the deep-seated equivalents of volcanic 

 rocks. The wor^ hypogenic is sometimes used in this sense. 



Poicilitic See Lustre-mottling. Also applied to the New Bed Sandstone series. 



Polygenous A word applied by some authors to clastic rocks which are com- 

 posed of fragments derived from two or more different rocks. Used in 

 contradistinction to monogenous. Syn. Polymikt. 



Polymikt (Ger.)See Polygenous. 



Polymorphism The property possessed by some substances of assuming two 

 or more molecular structures. If the bodies crystallize, this difference is 

 expressed in the different parametral values of the axes of the several 

 crystalline forms. These bodies may be termed, with Tschermak, hetero- 

 morphous varieties of the same polymorphous substance. According to the 

 number of heteromorphous varieties it is capable of assuming, a substance 

 is dimorphous, trimorphous, &c. Carbonate of lime and titanic acid are 

 examples of dimorphism and trimorphism respectively, the former 

 occurring in nature as calcite and arragonite, the latter as rutile, anatase, 

 and brookite. Syn. Pleomorphism. 



Polysynthetie Twinning" Twinning in lamellae ; lamellar twinning ; twin- 

 lamellation ; twin-striation. 



Porodine A term applied by Breithaupt to rocks which are composed of 

 amorphous material and which have been produced by slow consolidation 

 from a gelatinous condition ; as for example, opal. 



Porphyritic This term is generally applied to any rock in which one or more 

 of the mineral components dominate in size over the remainder, the latter 

 constituting the yroundmass. Eosenbusch, however, proposes to define 

 " porphyritic structure " as that in which there is a recurrence of similar 

 minerals, due to the rock having consolidated in two distinct phases. 



Positive (optic) See Elasticity of Crystals. 

 Pressure-metamorphism See Mechanical metamorphism. 



Prism In crystallography this form may be treated as a special case of the 

 pyramid, namely, that in which pyramidal planes cut the vertical axis at an 

 infinite distance. This being the case, each kind of pyramid furnishes a 

 prism, and so we have proto-, deutero-, trito-, macro-, brachy-, ortho-, and 

 clino-priams. See Pyramid. 



Prismatic As applied to minerals, a term indicating that they occur in acicular 

 or columnar forms, such minerals giving lath-shaped sections. As applied 

 to rocks it refers to their mode of jointing. 



