MINERALOGY. 



Pitch-ore, or Pitch-llaide, an ore of Uranium. See 

 Uranium, and Pitch-ore. 



PiTCH-STOXE, Re/mite, and Petro-filex refinite, Fr. 

 This mineral is named from the ftriking refemblance which 

 fome varieties have in colour and luilre to pitch. Its pre- 

 vailing colours are dark-green, from which it pafles into 

 black, grey, and blue, to brown and red. It is feebly 

 tranflucent, and has a gliilening or fhining \'itreo-refmous 

 luftre. It occurs maffive in veins and beds of confiderable 

 magnitude. Pitch-ftone is fometimes columnar, and fome- 

 times in thick and wedge-fhaped concretions, or in fomewhat 

 globular or curved lamellar dillinft concretions, and fome- 

 times it has a flaty ftructure. The fracture is more or lefs 

 perfeftly conchoidal, or paffing into fplinter)-. The frag- 

 ments are angular and (harp-edged. It fcratches glafs, is 

 rather eafily frangible, and is fufible into a grey fpongy 

 enamel. Some varieties of this mineral, however, fufe with 

 great difficulty by the blow-pipe. The fpecitic gravity of 

 pitch-ftone is about 2.3. According to Klaproth, its con- 

 ftituent parts are, 



Pitch-ftone of MeifTon. 



Silex 



Alumine 



Lime 



Oxyd of iron 



Oxyd of manganefe 



Soda 



Water 



73.00 



14.50 



1. 00 



1. 00 



10 



1-75 

 8.50 



99.85 



Pitch-ftone is found in various parts of Scotland, and in 

 the iflands of Rum, Egg, and Arran. It bears a near 

 refemblance to obfidian, or volcanic glafs, into which it 

 appears to pafs, and alfo into pearl-ftone. It is regarded 

 by many geologifts as a volcanic product. Like bafalt, it 

 interfects rocks of different formations, from granite to 

 fand-ftone, and is intimately aflbciated with bafaltic rocks. 



Plasma has generally a dullilh-green colour, with yel- 

 lowifh or whitilh dots, a gliftening luftre, a conchoidal 

 fracture, is tranflucent, and rather harder than quartz. It 

 confifts of about ninety -feven parts of filex, and appears to 

 be properly a green variety of flint. It was confidered by 

 the Romans as a gem, and figures were engraved upon it. 

 Moft of the fpecimens in collections were found in the ruins 

 of Rome. 



Platina. See Platixa. 



Pleoxaste. See Ceylaxite, Addenda. 



Plumbago. Graphite or black-lead. See Plumbago, 

 and Graphite. 



Porcelaix-clay. (See Porcelain.) The porcelain- 

 clay of Cornwall occurs in beds of coniiderable thicknefs 

 in the parifti of St. Stephen's. According to Wedgwood, 

 it confifts of fixty parts alumine, and forty of filex. Porce- 

 lain clay originates from the decompofition of granitic rocks, 

 abounding in felfpar, and frequently contains portions of 

 quartz and mica. The abfence of iron in porcelain-clay is 

 what conftitutes one of its moft effential properties, — that 

 of remaining white after burning. The Kaolin or Chinefe 

 porcelain-clay contains a much larger portion of filex than 

 of alumine. 



Porcellaxite, or Porcelain Jafper, (fee Jasper,) 

 appears to be formed accidentally by fires in coal-mines, 

 w hich have indurated and femivitnfied beds of coal-ftiale or 

 flate-clay. 



Pot-stoxe, Lapis ollaris ; Talc olaire, Haiiy. (See 

 Vol. XXXIX. 



Pot-stone.) This mineral appears to be indurated talc, 

 paffing mto ierpentme ; mth the latter rock, it is frequently 

 afTociated. It has a curved and undulatinglv-lameUar 

 ftruclure, paffing into flaty. It is ver)- foft, feftile, and 

 greafy to the feel, is tranflucent on the edges, and aff'ords 

 a white-coloured ftreak. It is infufible before the blow- 

 The analyfis of this mineral given by Tromfdorf is. 



pipe. 



Silex 

 Magnefia 

 Oxyd of iron 

 Carbonic acid 

 Water 



39 

 16 

 10 



20 

 10 



Near In\erary there is a quarry of talcous flate, fome of 

 which appears to pafs into pot-ftone, and can be turned in 

 the lathe ; of this ftone, Inverary Caftle is built. Pot-ftone 

 was extenfively ufed in Upper Egypt for culinarj- vefl'els. 

 They were found to refift the aftion of fire, and did not 

 communicate any tafte to the food boiled in them. Quar- 

 ries of pot-ftone were worked on the banks of the Lake 

 of Como from the beginning of the Chriftian era to the 

 year 161 8, when the mountain fell down on the 25th of 

 Auguft, and deftroyed the neighbouring town of Pleurs. 

 This town had annually raifed ftone from thefe quarries 

 to the value of 60,000 ducats. Pot-ftone is aKo ufed for 

 lining ovens and furnaces, and is remarkably durable. 



Prase, Quarz hyalin vert obfcur, Haiiy, is tranflucent 

 green quartz, the green colour being derived from an inti- 

 mate intermixture of quartz and actinolite. See Quartz> 

 and Prasius. 



Precious Garnet. See Garxet. 



Prehxite. (See Prehnite.) This mineral is divided 

 into two fub-fpecies by Mr. Jamefon ; foliated preh- 

 nite and fibrous prehnite. The prevailing colours are, 

 green, greenifli-white, and yellowifti-white. It occurs both, 

 maffive and cryftaUized in obhque four-fided tables, or in 

 fix or eight-fided tables ; alfo in four-fided and fix-fided 

 prifms. The cryftals are generally attached by their 

 lateral plains, and form diverging groups. The cleavage is 

 fingle, parallel to the fhort diagonal of a rhomboidal prifm, 

 the planes of which are inclined at angles of 103° and 77°. 

 The luftre is fliining and pearly ; it is tranflucent or tranf- 

 parent, fcratches glafs with difficulty, and intumefces before 

 the blow-pipe. Prehnite was difcovered by Mr. Bakewell 

 in a rock bafaltic amygdaloid, near Berkley in Gloucef- 

 terfliire, accompanied with green earth and maffive lamellar 

 prehnite, or kenpholite. This is the only Enghfti locahty 

 of thefe minerals at prefent known. It was firft brought 

 from the Cape of Good Hope. 



Fibrous Prehnite has a dehcate fibrous ftructure, either 

 ftraight, diverging, or ftellular. According to Langier, its 

 conftituent parts are. 



Both fub-fpecies of prelinite bear a near refemblance to 

 zeolites, but they do not gelatinize snxh acids, and they be- 

 come electric by heating. 



Pumice. See Pumice-stone, and Volcanic Produds. 



PvoiTii Schorltte, or Schorlaceous Beryl. (See 

 4 D Pycxite.) 



