110 PHYSIOGRAPHY. 



Pitchstone. 



4. Pitchstone forms mountain masses, and is generally in close con- 

 nexion with porphyry. Many of the other varieties occur under similar 

 circumstances. It is often the paste of certain kinds of porphyry, con- 

 taining imbedded crystals of other minerals; and in a similar manner, 

 Obsidian, Pearlstone, and Pumice, form, each their porphyry, denomina- 

 ted after the kind of paste which contains the crystals. All these varie- 

 ties occur also in beds in sandstone, in which it has been observed that 

 in some places they lie regularly between the strata, or abruptly as- 

 sume another situation, interrupt the strata, and then appear in the shape 

 of regular veins. Several of the pitchstoce veins in red sandstone seem 

 to have the same origin ; but it cannot be determined whether this also 

 is the case in similar veins in granite, where they likewise occur. Ob- 

 sidian frequently occurs in grains. Pumice, and several of the other 

 varieties of Pitchstone, are products of active volcanoes. 



5. Considerable masses of very distinct Pitchstone occur on the foot 

 of the Saxon metalliferous mountains at Meissen, also at Planitz near 

 Zwickaw, passing into Obsidian in the isle ot Arran. Pearlstone, inclu- 

 ding grains of Obsidian, is found between Tokay and Keresztur, and at 

 Glashiltte near Schemnitz in Hungary, at Cabo de Gata in Spain, near 

 Ochotzk in Siberia, &c. Obsidian is very frequent in Iceland, where it 

 exists in grains, angular pieces and beds ; it is also found at Schemnitz 

 and Glashatte in Hungary, of a green color at Moidauthein in Bohemia, 

 and shewing every stage of the passage into Pumice, in the Lipari isl- 

 ands, also in Teneriffe, Peru, and New Spain. Pumice occurs at Vesu- 

 vius, in Iscbia, the Lipari islands, and several islands of the Grecian 

 Archipelago, in TeneiirTe; near Tokay, Sclieirnitz, and other places in 

 Hungary; near Andernach, and the lake of Laach on the Rhine; in 

 Quito and Mexico, &c In several of these countries, also, it is met 

 with in conglomerates. 



6. Obsidian is employed for mirrors, vases, snuffboxes, &c. ; in Mex- 

 ico and the island of Ascension, very sharp edged fragments are used as 

 tools and weapons. Pumice yields a well known material for grinding 

 and polishing, and is also employed as a filtering stone. 



PITCHY IRON-ORE. (See Iron Sinter and Triplite.) 

 PLASMA. (See Quartz.) 

 PLEONASTE. (See Spinel.) 



