21 6 



which is wanting in the modern lavas. When 

 the basis of the lavas of the Malpays changes 



lagen) of porphyry. The firsth primitive, and found in sub- 

 ordinate strata in the gneiss, and the mica-slate (Isaac at 

 Freyberg). The second alternates with syenit : it is older 

 than grauwakke, and belongs most probably already to the 

 transition mountains ( uebergangs gebirgej. It contains beds of 

 pitchstone and obsidian, and even granular limestone, of which 

 we see instances near Meissen in Saxony : it is extremely 

 rich in metals, and is found in Mexico at (Guanaxuato, 

 Regla, &c), in Norway, m Sweden, and at Schemnitz in 

 Hungary. The porphyry cf Norway covers, near Skeen, 

 grauwakke and mandelstein ; it encloses crystals of quartz* 

 Near Holmestrandt, a bed of basalt, which abounds in augit, 

 is interposed among the transition porphyry. The rock of 

 Schemnitz (the saxum metalliferum ofFerber and Born), which 

 lies on the thonschiefer, is destitute of quartz, and contains 

 hornblende and common feldspar. It is this second formation 

 of porphyry which appears to have been the centre of the 

 oldest volcanic revolutions. The third formation belongs to 

 the ancient sandstone (todtesliegende), which seems as a basis 

 to the alpine limestone (alpen-kalkstein or zechstein) : it 

 contains mandelstein (amygdaloides) mixed with agate (at 

 Oberstein, in the Palatinate), and sometimes covers (in Thu- 

 ringia) strata of coal. The fourth formation of porphyries is 

 trappean, destitute of quartz, and, especially in America, 

 often mixed with olivin and augit ; it accompanies basalts, 

 greenstone, and phonolites (Chimborazo, the province de los 

 Pastos, Drachenfels near Bonn, Puy-de-D6me). The classi- 

 fication of the porphyries is accompanied with great difficul- 

 culties. Granite, gneiss, mica slate or micaceous schist, thons- 

 chiefer, and chloritschiefer, forms a series, in which each rock 

 is connected with that which precedes it. The porphyries, on 

 the contrary, are found, as it were, isolated in the geognosti- 

 ral svstem ; they offer transitions into each other, biit not 



