443 

 Petrogeny That department of geology which treats of the formation of rocks. 



Petrography The science which treats of the mineralogical composition and 

 minute structure of rocks. 



Petrology (Ger. G-esteinskunde, Gesteinslehre) The science which treats of 

 rocks. Syn. Petralogy. 



PetrosilCX A term employed by French petrographers to denote the crypto- 

 crystalline groundmass of many felsites and porphyries, the chemical 

 composition of which indicates an aggregate of quartz and felspar. 



Pfloek-Struetur (Ger.) The word used by Stelzner (N.J.B.B.,11., 1882, p. 369) 

 to designate the micro-structure of some varieties of melilite. Sections 

 of this mineral cut at right angles to the basal plane are sometimes seen 

 under the microscope to be traversed transversely by curious bolt-, peg-, 

 or oar-shaped bodies. There seems to be some doubt as to the nature of 

 these Pfldcke. In fact, Stelzner himself could not decide whether to 

 regard them as solid bodies or cavities. 



Phanerocrystalline A word used by some authors to embrace both macro- 

 and microcrystalline (q.v.). It is thus opposed in meaning to crypto- 

 crystalline, 



Phane'rogene A French term for coarsely granular ; used in contradistinction 

 to adelogene (q.v.). 



PhaneromerOUS A term applied to coarse crystalline rocks. See Crypto- 

 merous. 



ph aS 9 A term used in speaking of the consolidation of a rock to denote a 

 periodicity of crystallization. The first phase is that in which the 

 porphyritic minerals are separated ; the second, that in which the ground- 

 mass consolidates. See " Recurrence of phase." 



PhytOgenic Applied to rocks which are composed of the remains of vegetable 

 matter. 



PilotaxitiC The name given by Kosenbusch (Mikros. Physiog. der massig. Gest., 

 p. 466) to a holocrystalline structure especially characteristic of certain 

 porphyrites and basalts. The groundmass of these rocks consists essen- 

 tially of slender laths and microlites of felspar in felted aggregation, and 

 often presents fluxion phenomena. If, in addition, films of glass are 

 present, the structure is said to be hyalopilitic. The normal structure of 

 the andesites is the hyalopilitic (Eosenbusch, /. c., p. 673). 



Pisolitie The name given to an oolitic structure when the spheroidal concre- 

 tions are at least of the size of a pea, as in the so-called Erbsenstein of 

 Carlsbad and the Pea-grit of the Cotteswold Hills. See Oolitic. 



Plan-parallel-Structur (Ger.') That structure of rocks which is produced by a 

 parallel arrangement of minerals having a tabular, lamellar, or scaly 

 development (e.g., felspar, mica, talc). It may be original (lamination) or 

 superinduced (schistosity or foliation). 



Pleochroic Borders (Ger. pleoohroitisehe Hofe) In some, usually colourless, 

 minerals a zone of strong pleochroism is found around certain mineral- 

 inclusions. The phenomenon is often observed in andalusite. In 

 polarized light this mineral appears colourless in certain positions ; on 

 rotation, however, a number of bright yellow spots spring into view, again 

 disappearing on the rotation being continued. Mica often behaves 

 similarly. 



