217 



from pitchstone to obsidian, the color is paler, 

 and mixed with gray ; in this case, the feldspar 

 passes by imperceptible gradations from the 

 common to the vitreous. Sometimes both va- 

 rieties meet in the same fragment, as we ob- 

 served also in the trappean porphyries of the 

 valley of Mexico. The feldsparry lavas of the 

 Peak, of a much less black color than those of 

 Arso, in the isle of Ischia, whiten at the edge 

 of the crater from the effect of the acid vapors ; 

 but their inside is no way deprived of color like 

 that of the feldsparry lavas of the solfatara at 

 Naples, which perfectly resemble the trappean 



into the substances on which they repose. (Buch, Geognosi. 

 Beob. t. i, s. 56.) As in the course of this work volcanic and 

 nonvolcanic porphyries may often occur, it appears to me in- 

 dispensible, to exhibit the general table of the formation 

 traced by the illustrious chief of the Freyberg school, from 

 his own observation, those of Von Buch, Esmark, and Fries- 

 leben, and mine. The great divisions, which are suscepti- 

 ble of much improvement, are independent of any hypothesis 

 on the origin of porphyries, as they relate only to position, 

 superposition, and relative age. The four formations just 

 described, may be distinguished by the names of primitive 

 porphyries ( urporphyre ), of transition porphyries (ueber gangs- 

 porphyrej , secondary porphyries (flcezporphyrej, and trappean 

 porphyries (trapporphyre) . If we confound the second 

 and fourth of these formations under the common name of 

 porphyry -lav as, we throw geognosy back into the obscurity 

 from which it is scarce freed : we might as well class gneiss, 

 mica-slate, and thonschiefer, under the general name of 

 laminar and schistose rocks. 



