114 
VOLCANIC COMBUSTION. 
crater is an infallible indication of the proximity of a new 
eruption. 
1 might terminate these geological sketches by enquiring 
into the nature of the combustible which has fed for so many 
thousands of years the fire of the peak of Teneriffe ; — I might 
examine whether it be sodium or potassium, the metallic 
basis of some earth, carburet of hydrogen, or pure sulphur 
combined with iron, that burns in the volcano; — but wishinrr 
to limit myself to what may be the object of direct obsei'- 
vation, I shall not take upon me to solve a problem for which 
we have not yet sufficient data. We know not whether we 
may conclude, from the enormous quantity of sulphur con- 
tained in the crater of the Peak, that it is this substance 
which keeps up the heat of the volcano; or whether the 
fire, fed by some combustible of an tmknown nature, effects 
merely the sublimation of the sulphur. What we learn 
from observation is, that in craters which are still burning, 
sulphur is very rare ; while all the ancient volcanoes end in 
becoming sulphur-pits. Wo might presume that, in the 
former, the sulphur is combined, with oxygen, while, in 
the latter, it is merely sublimated; for nothing hitherto 
authorises us to admit that it is formed in the interior of 
volcanoes, like ammonia and the neutral salts. When we 
were yet unacquainted with sulptmr, except as disseminated 
in the murktiferous gypsum and in the Alpine limestone, 
we were almost forced to the belief, that in every part of 
the globe the volcanic fire acted on rocks of secondary for- 
mation; but recent observations have proved that sulphur 
exists in great abundance in those primitive rocks which so 
many phenomena indicate as the centre of the volcanic 
action. Near Alausi, at the back of the Andes of Quito, I 
found an immense quantity in a bed of quartz, which formed 
a layer of mica-slate. This fact is the more important, as it 
is in strict conformity with the conclusions deduced from the 
observation of those fragments of ancient rocks whicli are 
thrown out intact by volcanoes. 
We have just considered the island of Tenerilfe merely in 
a geological point of view ; wc have seen the Peak towering 
amid fractured strata of basalt and mandelstein; let us 
examine how these fused masses have been gradually 
