M. Godon’s mineralogical observations. 139 
alternates with gneiss and micaceous schistus, as belonging to anoth- 
er order of rocks, and to another formation. ‘Time will determine 
what is the value of this distinction. 
18. Felsparoid presents in general no distinct stratification. _ It in- 
cludes often veins of amorphous hyaline quartz, sometimes full of cav- 
ities, where this mineral exists with a crystalline shape. (Commonly 
var. prismoid). It falls most often into polyedrical fragments, the faces 
of which are often very smooth. Exposed to the contact of the air 
it moulders ; and, when the felspar is in great proportion, its whole 
surface consists of an argillaceous crust. Ina fresh state the several 
kinds of this rock are a of a fine 5 Pola even superior to that 
of granite. 
_ When the siemnenis of 3 felsparoid ae to be entwined together i in 
a confused state, so as to take the appearance of a homogeneous paste, 
it assumes the characters ascribed to petrosilex. [Brushhill turnpike]. 
When crystals of felspar remain imbedded in this uniform paste, it 
constitutes the porphyry about to be desgribed among the species of 
petrosilex. The transition of felsparoid to petrosilex and_porphyritic 
petrosilexis frequently observed in the compass of the present obser- 
vations. [Milton 0 deals o, Malden 0}. 
Petrosilex. 
19. Mictslogss bave generally considered petrosilex as a sim- 
ple mineral, and respectable authorities agree in placing it among the 
varieties of felspar. But if we consider, that most of the great mine- 
ral masses of the globe consist of an aggregation of distinct species ; 
that petrosilex accompanies these rocks, and forms itself vast masses 
and even mountains ; thatamong the specimens of it, which are found 
im nature, or seen in collections, hardly one is ever observed ‘perfect- 
ly free of foreign substanses visible to the eye ; we may entertain’ 
/ 
Bn. 
