r o E 



gemmule the power of locomotion 

 is conferred, until it has found for 

 itself a proper habitation ; this 

 chosen, it there fixes itself and 

 there continues for the remaining 

 period of its existence. 



POEI'PHEEOTJS. Possessing pores ; 

 animals belonging to the class pori- 

 phera. The alimentary apparatus 

 of poripJierous animals approaches 

 the nearest to that of plants. 



PO'BPHYEY. (from tropcfrvpa, purple, 

 Gr.) Porphyry has been so called 

 in reference to the purple, or 

 reddish, colour so commonly per- 

 ceptible in it. Generally, any form 

 of rock in which one or more 

 minerals are scattered through an 

 earthy or compact base. Porphyry 

 has generally a compact texture. 

 Sometimes it is composed of tabu- 

 lar, columnar, or globular distinct 

 concretions ; and not unfrequently 

 it is traversed by numerous seams 

 and rents. There are many va- 

 rieties of porphyry, named accord- 

 ing to the base of each, as Petro- 

 siliceous Porphyry, Felspar Por- 

 phyry, Clinkstone Porphyry, Ar- 

 gillaceous Porphyry, &c. Porphyry 

 occurs in enormous masses ; at the 

 head of Glen Ptarmagan, a cliff of 

 porphyry fifteen hundred feet in 

 height, in shape resembling an 

 oblique truncated pyramid, passes 

 through granite. In some in- 

 stances porphyry is, beyond all 

 question, a volcanic formation. 

 Near Christiana, in Norway, an 

 immense mass of porphyry, from 

 1600 to 2000 feet in thickness, 

 covers beds of gneiss, limestone, 

 and greywacke. Dykes of por- 

 phyry cutting through the sub- 

 jacent rocks indisputably prove the 

 volcanic character of this immense 

 mass. 



POEPHYEI'TIC. } Resembling por- 



PORPHYKA'CEOTTS. ) phyry; contain- 

 ing porphyry ; composed of a 

 compact homogeneous rock, in 

 which distinct crystals or grains 



L 365 ] POT 



are imbedded : the compact stone 

 is called the base, and sometimes 

 the paste. The base, or paste, is 

 generally felspar. 



POEI'TES. A genus of stony polypi- 

 fers, fixed; ramified; or lobated 

 and obtuse; the outer surface 

 everywhere stellated. 



PO'ETLAND BEDS. A marine forma- 

 tion, occurring in the Isle of Port- 

 land and in Wiltshire. These beds 

 consist of coarse shelly limestone, 

 fine grained white limestone, and 

 compact limestone, all possessing 

 an oolitic structure ; and beds of 

 chert. The Portland beds lie im- 

 mediately under the Purbeck beds, 

 and above the Kimmeridge clay. 

 They constitute the uppermost 

 members of the oolite group, and 

 abound in ammonites, trigoniae, &c. 



PO'ETLAND L'IMESTONE. ") One of the 



PO'ETLAND OOLITE. j members of 

 the Portland beds ; a marine oolitic 

 formation, obtained principally 

 from Portland, whence the name, 

 and used in building. The Port- 

 land limestone abounds in trigoniae, 

 ammonites, pernae, pleurotomarias, 

 and other marine shells. 



PO'ETLAND SANDS. A name given to 

 one of the divisions of the Portland" 

 oolite. These beds consist of white 

 and green sand and sandstone, with 

 concretionary masses of grit ; the 

 lower strata argillaceous. Mantell. 



POST. A north of England term for 

 any bed of firm rock; generally 

 applied to sandstone. 



POST PLF/IOCENE. See Quaternary. 



PO'TASH. An alkali obtained by the 

 incineration of vegetables, or the 

 woody parts of plants that do 

 not grow near the sea. The water 

 in which the ashes are washed is 

 evaporated in iron pots, from which 

 circumstance it was called potash. 

 There are few, if any, of the inferior 

 stratified rocks without potash ; 

 and, viewing them in the mass, 

 potash may be considered as con- 

 stituting five or six per cent, of 



