P E I 



[367] 



P E I 



sparks with, steel. Specific gravity 

 from 2-60 to 2*94. Hardness 6 

 to 7. Veins of prehnite occur in 

 Cornwall ; they are very irregular, 

 both in size and direction. As- 

 bestus and stilbite have been found 

 in the veins with the prehnite. 



PRIMARY. 1 . In astronomy, the planet 

 about which a satellite revolves. 

 The planetary bodies which revolve 

 round the sun, and not round any 

 other body, are called primary 

 planets, or primaries. Some of 

 the planets are accompanied by 

 satellites or moons, which revolve 

 round their primary, in a manner 

 similar to the revolution of our 

 moon. 



2. A term applied to rocks or 

 strata, because it was supposed, 

 from the absence of fossil remains, 

 that they were formed before ani- 

 mals and vegetables ; as well as 

 that they were the first rocks 

 formed. Sir C. Lyell proposes to 

 substitute the word hypogene for 

 primary. 



PRIMARY FORM. In mineralogy, that 

 form to which minerals may be 

 reduced by cleavage, and which is 

 no longer changed by continued 

 cleavage. If a mineral can be 

 cleaved in directions which produce 

 only one particular form that form 

 is denominated its primary. The 

 whole number of primary forms 

 are comprised in the regular tetra- 

 hedron, the cube, the rhombic do- 

 decahedron, the octahedron, the 

 six-sided prism, and the parallele- 

 piped . 



PRIMARY LIMES'TONE. This, says 

 Jukes, instead of primary, ought to 

 be simply called altered limestone. 

 It is always highly crystalline, 

 generally has lost all appearance of 

 bedding, and is a granular crystal- 

 line carbonate of lime. 



PRI'MARY STRATA. The primary strata 

 are defined above by the old red 

 sandstone ; and when that is absent, 

 by the carboniferous limestone; 



below, they usually rest, but some- 

 times unconformably, upon granite. 

 They consist, in a great measure, 

 of mechanical aggregates, compara- 

 ble with sandstones and clays, but 

 yet generally distinguishable by 

 superior hardness, and somewhat 

 of a crystalline structure in mass, 

 or texture in detail, from the se- 

 condary rocks. In the secondary 

 rocks there is more variety of aren- 

 aceous and calcareous members. 

 In the tertiary strata loose sands, 

 marls, and clays abound, while 

 these scarcely occur at all among 

 the primary rocks. Prof. Phillips. 



PRIMITIVE. A term applied to cer- 

 tain rocks, from the circumstance 

 of no fossil remains of animals or 

 vegetables, nor any fragments of 

 other rocks, being found in them. 

 The term has given way to what is 

 considered a more appropriate one, 

 namely, primary, and primary has 

 given way to Palaeozoic. 



PRISM. (Trpiffjua, Gr. prisme, Pr. 

 prisma, It.) A solid figure, the 

 ends whereof are parallel, equal, 

 and similar plane figures, and the 

 sides which connect the ends are 

 parallelograms. Prisms take par- 

 ticular names from the figure of 

 their bases or ends, namely, triangu- 

 lar, square, rectangular, pentagonal, 

 hexagonal, &c. 



" A prism is rarely found having 

 only three, very commonly four, 

 six, eight, or even more sides ; the 

 sides, or lateral planes, surround 

 its axis, which is an imaginary 

 line passing down the middle of 

 the prism, from the centre of the 

 upper terminal plane to the centre 

 of the lower ; the terminal planes 

 being also called the bases. But 

 prisms are found both very long 

 and very short; when long, and 

 the crystals slender and curved, 

 they are termed capillary, when 

 straight, acicular ; when the prism 

 is short, the crystal is said to be 

 tabular" Phillips. 



