215 



rendered very probable *, that these crystals are 

 formed only in the currents which flow either 

 from the crater itself, or very near it's brinks we 

 must not be surprised, if we do not find them in 

 the lavas of the Peak, which are almost all owing 

 to lateral eruptions, and which consequently 

 have been exposed to an enormous pressure in 

 the interior of the volcano. 



In the plain of Retama, the basaltic lavas dis- 

 appear under heaps of ashes, and pumice stone 

 reduced to powder. Thence to the summit, 

 from 1500 to 1900 toises in height, the volcano 

 exhibits only vitreous lava with basis of pitch- 

 stone •jf and obsidian. These lavas, destitute 

 of hornblende and mica, are of a blackish brown, 

 often varying to the deepest olive green. They 

 contain large crystals of feldspar, which are not 

 fissured, and seldom vitreous. The analogy of 

 those decidedly volcanic masses with the resinit 

 porphyries ^ of the valley of Tribisch in Saxony 

 is very remarkable; but the latter, which 

 belong to a very extended and metalliferous 

 formation of porphyry often contain quartz, 



* Leopold yon Buck, Oeognosiische Beob. t. 2, s. 221. Gil- 

 bert's Ann. t. 6, s. 53, The existence of leucites (amphigenes) 

 at Arendahl in Norway, in Scotland, in the Pyrenees, in 

 Transylvania, in Mexico, does not rest on very accurate 

 observations. 



t Petrosilex r£sinite. Haiiy. 



% Pechstein-porphyr. Werner. 



% We can now distinguish four formations (hauptnieder- 



