442 



Outcrop The term applied to the edge of a stratum as it is exposed at the 

 surface of the earth. Syn. Basset. 



Oxygen-ratios (G-er. Sauerstoff-quotienten) G. Bischof first introduced the 

 method of quoting rock or mineral analyses by oxygen ratios. These are 

 obtained by dividing the oxygen of the bases by that of the silica. Thus : 



xO (of monoxide) + yO (of sesquioxide) 

 zO (of silica) 



PalaeovolcaniC Rosenbusch (Physiog. der Massig. Gest., 1886, p. 6) subdivides 

 the group of volcanic rocks into paleovolcanic and neovolcanic, according 

 as they are of pretertiary or postcretaceous age. 



Panidiomorphic Applied to rocks in which all the minerals present are idio- 

 morphic (q.v.). 



Panniform A term sometimes used to designate the corrugated aspect of con- 

 solidated lava, which often sirnilates the appearance of rumpled cloth. 



Paradase See Joint and Fault. 



Paragenesis This term, introduced by Breithaupt (Die Paragenesis der 

 Mineralien, Freiberg, 1849), embraces the laws regulating the association 

 of minerals. 



Parallel Structure That structure of rocks in which the mineral components 

 or the differently constituted parts of the rock are arranged parallel to one 

 another, the result being a more or less well-defined banding. Such 

 parallelism may be due (1) to original deposition (lamination), (2) to differ- 

 ential movement before consolidation (fluxion structure), (3) to subsequent 

 deformation by earth-stresses. 



Parameter The intercepts (measured from the origin) cut off from the axes of 

 a crystal by one of its faces constitute the parameter of the latter. 



Paramorphism The name given by Stein to those pseudomorphous changes 

 which take place without loss or gain of mineral substance. Synonymous 

 with this term is the word allomorphism, proposed by Dana. 



Paroptesis A term suggested by Kinahan (Geology of Ireland, 1878) for 

 contact-metamorphism. 



Parting A thin layer, coincident with the bedding-plane, of some soft rock 

 between two harder beds. Along the separation thus produced the beds 

 are said to " part." N.B. A thin layer of a hard rock between two softer 

 ones is generally termed a seam. 



The same word is used by G. H. Williams (Bull, of the U.S. Geol. 

 Survey, No. 28, p. 22) to designate the lamellar separation (parallel to 

 the orthopinacoid) of diallage. (Ger. Absonderung.) 



Pegmatitic A structure produced by an intimate intergrowth of two minerals 

 of simultaneous formation. The pegmatitic intergrowth of quartz and 

 felspar is familiar to all in the so-called graphic granite. 



Pelitic (Naumann) Having a texture like that of dried mud. 



Perimorphism Scheerer termed those crystals perimorphic which consist 

 merely of a thin rind, the interior being filled with other minerals. 



Perlitie A rock structure especially characteristic of the rock known as 

 perlite ; also found in other vitreous, but never in holocrystalline, 

 rocks. Thin sections of rocks possessing this structure show under the 

 microscope a series of more or less imperfect curvilinear cracks. Besides 

 these, rectilinear cracks are often present. 



