OBDEftVl. PRISMATIC FELD-SPAR. 



distinct cleavage, and compound masses of small or impalp- 

 able and strongly connected individuals. Common Fel- 

 spar contains varieties of all the species enumerated above, 

 except perhaps Labradorite, which forms a great proportion 

 of compact Felspar. From the latter subspecies, Clinkstone, 

 which is commonly a mixed mineral, and forms the mass 

 of porphyry slate, was distinguished as a particular species, 

 and moreover Var\ol\te, consisting of small globular masses 

 imbedded in a mixed rock. It has not been exactly ascer- 

 tained to what species Clinkstone and Variolite belong, nor 

 is it possible to settle at present any thing in this respect. 

 Imbedded crystals of considerable degrees of transparency 

 in porphyry slate, occurring also in various other trachytic 

 and volcanic rocks, were called glassy Felspar. Their forms 

 are generally hemi-prismatic, as in the examples from 

 Drachenfels and Ischia ; but sometimes also tetarto-pris- 

 matic varieties are observed in similar rocks, as in those 

 from the Gallopago islands, brought home by Captain 

 BASIL HALL. Ice-spar occurs in white transparent crys- 

 tals, greatly resembling Adularia and glassy Felspar, but 

 implanted in the drusy cavities of rocks ejected by Mount 

 Vesuvius. However complicated this division may be, it 

 is not all that was to be considered in the various kinds of 

 Felspar. In regard to the particular state in which the 

 varieties of common Felspar occur, those which are more 

 or less decomposed, were designated by the denomination 

 of earthy common Felspar, and considered as a particular 

 subspecies. If the decomposition has arrived at its limits, 

 so that the whole is converted into a more or less firmly 

 coherent powder, Porcelain-Earth is formed. This substance 

 is not only considered as a distinct species in the Werneriari 

 system, but it is not even contained in the same genus with: 

 the remaining subspecies. It is possible that porcelain earth 

 arises from the decomposition of several species of the pre- 

 sent genus, but we do not possess sufficient information in 

 this respect. Professor FUCHS is of opinion that the porce- 

 lain-earth from Passau is produced by the decomposition of 



