O T 



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P R I 



grey colour, with various shades, and 

 often spotted. Its fracture curved, and, 

 sometimes, almost foliated. Specific gra- 

 vity from 2-8 to 3'2. It emits an argil- 

 laceous odour when breathed on. From 

 its being formed into culinary vessels it 

 has obtained its name. From an analysis 

 by Wiegleb, the potstone of Corno, in 

 Lombardy, where it occurs in great abun- 

 dance, consists of magnesia 38*0, silica 

 38'0, alumina 7'0, iron 15 0, carbonate of 

 lime I'O, fluoric acid 1*0. 



PO'TTERS' CLAY. A variety of clay, of a 

 reddish or grey colour, which becomes 

 red when heated. That used in our pot- 

 teries for making coarse red ware comes 

 chiefly from Devonshire. It is exceed- 

 ingly infusible, and contains a large pro- 

 portion of alumine. 



POZZUOLA'NA. Scoriae or volcanic ashes, 

 brought from Pozzuoli, a town in the bay 

 of Naples, and named therefrom. Poz- 

 zuolana is used, mixed with lime, for 

 making a Roman, or water-setting, ce- 

 ment. 



PRASE. The Prasem of Werner ; Quartz 

 hyalin vert obscur of Haiiy ; Quartz 

 prase of Brongniart. A leek- green trans- 

 lucent variety of rhombohedral quartz ; 

 lustre vitreous ; fracture splintery. Spe- 

 cific gravity 2*5. Prase appears to be 

 common quartz, coloured by actynolite or 

 epidote. 



PRA'SINOUS. (prasinus, Lat.) Of a light 

 green colour, inclining to yellow. 



PRECI'PITATE. A deposit, in a solid form, 

 of a body previously held in solution. 



PRECI'PITATE. (precipiter, Fr. precipi- 

 tare, It.) To throw down a substance 

 that had previously been held in solu- 

 tion. 



PRECIPITOUS, (from preeceps, Lat. pre- 

 cipitoso, It.) Steep ; headlong. 



PREDA'CEOUS. (from prada, Lat. booty.) 

 Living upon prey. 



PREHE'NSILE. (from prehendo, Lat. to 

 lay hold of.) That can lay hold of. The 

 prehensile portion of the tails of monkeys 

 is naked beneath. 



PRE'HNITE. A mineral thus named after 

 Colonel Prehn, who brought it from the 

 Cape of Good Hope. Prehnite is of a 

 green, grey, or white colour. It occurs 

 crystallized ; in granular, scopiform, and 

 stellular fibrous distinct concretions ; 

 massive and reniform. Its texture is fo- 

 liated. Fracture uneven. Internal lustre 

 pearly. It scratches glass, though feebly, 

 and gives sparks with steel. Specific 

 gravity from 2'60 to 2'94. Mr. Bake- 

 well was the first person who discovered 

 prehnite in England; he found it in 1816, 

 in amygdaloidal wacke, in Gloucester- 

 shire. It has since been found in the 

 basalt of Staffordshire. 



PRI'MARV. A term applied to rocks or 

 strata, because it was supposed, from the 

 absence of fossil remains, that they were 

 formed before animals and vegetables ; as 

 well as that they were the first rocks 

 formed. Mr. Lyell, with his wonted 

 acumen, proposes to substitute the word 

 hypogene for primary. It would ap- 

 pear, says Mr. Lyell, that the popular 

 nomenclature of geology, in reference to 

 the so called "primary'' rocks, is not 

 only imperfect, but in a great degree 

 founded on a false theory ; inasmuch as 

 some granites and granitic schists are of 

 origin posterior to many secondary rocks. 

 In other words, some primary forma- 

 tions can already be shown to be newer 

 than many secondary groups, a mani- 

 fest contradiction in terms. See Hypo- 

 gene. 



PRI'MARY ROCKS. These rocks were for- 

 merly called primitive, and will, in all 

 probability, hereafter be called hypo- 

 gene. They are of crystalline structure, 

 and manifestly owe their present state to 

 igneous agency. The primary rocks are 

 those which are older than the most 

 ancient European group, (grey wacke,) 

 in which distinct fossils have as yet been 

 discovered. Primary rocks are divisible 

 into two groups, the stratified and the 

 unstratified. The stratified group con- 

 sists of the rocks called gneiss, mica 

 schist, argillaceous schist, hornblende 

 schist, primary limestone, and some 

 others. The unstratified, or Plutonic, 

 is composed in a great measure of gra- 

 nite, and rocks closely allied to granite. 

 LyeWs Principles of Geology. 



PRI'MARY STRA'TA. The primary strata 

 are defined above by the old red sand- 

 stone ; and when that is absent, by the 

 carboniferous limestone ; below, they 

 usually rest, but sometimes unconforma- 

 bly, upon granite. They consist, in a 

 great measure, of mechanical aggregates, 

 comparable with sandstones and clays, 

 but yet generally distinguishable by su- 

 perior hardness, and somewhat of a crys- 

 talline structure in mass, or texture in 

 detail, from the secondary rocks. In the 

 secondary rocks there is more variety of 

 arenaceous 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 certain 

 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. With the con- 

 tinued advance of geological knowledge, 



