GEOLOGY OE TIIE TEAK. 
99 
Ischia, whiten at the edge of the crater from the effect of 
the acid vapours ; but internally they are not found to be 
colourless like that of the feldsparry lavas of the Solfatara 
at Naples, which perfectly resemble the trappean porphyries 
at the foot of Chimborazo. In the middle of the Malpays, 
at the height of the cavern of ice, we found among the 
vitreous lavas with pitch-stone and obsidian bases, blocks 
oi real greenish-gray, or mountain-green phonolite, with a 
smooth fracture, and divided into thin laminae, sonorous and 
vCe P edged. These masses were the same as the porphyr- 
schiefer of the mountain of Bilin in Bohemia ; we recognised 
m them small long crystals of vitreous feldspar. 
I his regular disposition of lithoid basaltic lava and feld- 
® Parry vitreous lava is analogous to the phenomena of all 
trappean mountains ; it reminds us of those phonolites lying 
m very ancient basalts, those close mixtures of augite and 
eldspar which cover the hills of wacke or porous amyg- 
aioids: but why are the porphyritie or feldsparry lavas 
0 the Peak found only on the summit of the volcano ? 
'Qul'-l we conclude from this position that they are of 
more recent formation than the lithoid basaltic lava, which 
contains olivine and augite ? I cannot admit this last hypo- 
lesis ; for lateral eruptions may have covered the feldsparry 
lucleus, at a period when the crater had ceased its activity. 
' osuvius also, we perceive small crystals of vitreous feld- 
jP ar on lv m the very ancient lavas of the Somma. These 
‘"^setting aside tlie leucite, very nearly resemble tlm 
pnonolitie ejections of the Peak of Tenerifte. In general, 
ue larther wo go back from the period of modern eruptions, 
P\ ore the currents increase both in size and extent, 
Requiring the character of rocks, by the regularity of their 
position by their division into parallel strata, or by their 
aependonee of the present form of the ground. 
1 . le I°ak of Teneriffe is, next to Lipari, the volcano that 
',P ro ^ed most obsidian. This abundance is the more 
U - °^ ier r pgions of the earth, in Iceland, in Hun- 
. jfj;. 111 dicwico, and in the kingdom of Quito, we meet with 
6. I? 118 on] / g rca t distances from burning volcanoes'. 
Sc th °- V are lettered over the fields in angular 
es j. or instance, near Popayan, in South America ; at 
times they form isolated rocks, as at Quinche, near 
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