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porphyries at the foot of Chimborazo. In the 

 middle of the Malpays, at the height of the ca- 

 vern of ice, we found among- the vitreous lavas 

 with the pitchstone and obsidian basis, blocks of 

 real greenish-gray, or mountain green phonolite, 

 with a smooth fracture, and divided into thin 

 laminae, sonorous and keen edged. These masses 

 were the same as the porphyrschiefer of the 

 mountain of Bilin in Bohemia ; we recognized 

 in them small long crystals of vitreous feldspar. 



This regular disposition of lithoid basaltic lava 

 and feldsparry vitreous lava is analogous to the 

 phenomena of all trappean mountains ; it re- 

 minds us of those phonolites lying in very an- 

 cient basalts, those intimate mixtures of augit 

 and feldspar which cover the hills of wakke or 

 porous amygdaloids : but why are the porphy- 

 ritic or feldsparry lavas of the Peak found only 

 on the summit of the mountain ? Should we 

 conclude from this position, that they are of a 

 more recent formation than the lithoid basaltic 

 lava, which contains olivine and augit ? I can- 

 not admit this last hypothesis ; for lateral erup- 

 tions may have covered the feldsparry nucleus, 

 at a period when the crater had ceased it's ac- 

 tivity. At Vesuvius also, we perceive small 

 crystals of vitreous feldspar only in the very an- 

 cient lavas of the Somma. These lavas, setting 

 aside the leucite, very nearly resemble the pho- 

 nolitic productions of the Peak of TenerifFe. In 



