340 



PHYSIOGRAPHY. 



CLASS II. 



2-00 

 0-00 

 0-00 



1-10 

 1-00 

 8-50 



1-60 

 0-50 

 4-50 



1-75. 

 0-00. 



o-oo. 



Silica 



Alumina 



Potash 



Soda 



Oxides of Iron 



and Manganese 

 Lime 

 Water 



Before the blowpipe these varieties melt with more or less 

 facility, according to the fusibility of their ingredients, into 

 a vesicular glass, or they yield an enamel. 



3. The geological relations of the present species are very 

 remarkable. Pitchstone forms mountain masses, and is gene- 

 rally in close connexion with porphyry. Many of the other 

 varieties occur in similar circumstances. It is often itself the 

 paste of certain kinds of porphyry, containing imbedded crys- 

 tals of other minerals ; and in a similar manner obsidian, pearl- 

 stone, and pumice, form each their porphyry, denominated 

 after the kind of paste which contains the crystals. All 

 these varieties occur also in beds in sandstone, in which 

 that remarkable fact has been observed, that in some places 

 they lie regularly between the strata, and abruptly assume 

 another situation, interrupt the strata, and then appear in 

 the shape of veins. Several of the pitchstone veins in red 

 sandstone seem to have the same origin ; but it cannot be 

 observed whether this also be the case in similar veins in 

 granite, where they likewise occur. Obsidian frequently 

 occurs in grains, like those mentioned above in Pearlstone. 

 Several of the varieties of empyrodox Quartz, and more 

 particularly Pumice, are products of active volcanoes. 



4. Some countries are rich in varieties of the present 

 species. Considerable masses of very distinct Pitchstone 

 occur on the foot of the Saxon metalliferous mountains at 

 Meissen, also at Planitz near -Zwickau, passing into Ob- 

 sidian in the isle of Arran. Pearlstone, including grains of 

 Obsidian, is found between Tokay and Keresztur, and at 

 Glashutte near Schenmitz in Hungary, at Cabo de Gata in 



