72 CAUSES OF GEOLOGICAL CHANGES. 
interval of only one hundred and fifty miles, has never ceased to 
burn. 
3. Volcanoes appear to be less numerous in the crust of the 
earth at present, than they were in the most ancient times: 
from whence it follows that the causes of their activity may 
be either exhausted, or repressed and overcome. 
Although the only burning mountains in Europe, the only ones 
that have burnt within the memory of man, are in Iceland, about 
the southern extremity of Italy, and on the Grecian Islands, it is 
certain that the time has been, when the same phenomena were 
exhibited in other parts of the continent, between Rome and 
Florence, and on the northern side of the Po, in Italy; in Hun- 
gary, on both sides of the Rhine above Cologne, in the southern 
part of France, and the eastern part of Spain. The appearances 
of the mountains in these countries taken in connexion with the 
nature of the substances lying round their bases, are such as to 
leave no doubt as to their having once been the seat of volcanic 
fires. The figure of a volcanic mountain is altogether peculiar as 
well as the material in most cases of which it is composed. 
Wherever a mountain is found in the form of a truncated cone, 
with a basin-shaped cavity upon its top, and beds of lava about 
its base, we need find no difficulty, although it may never have 
been known to emit either smoke or flame, in pronouncing re- 
specting its origin. Eminences having this figure and the struc- 
ture and composition of active volcanoes, occur in the countries 
mentioned, as well as in other parts of the globe, in places remote 
from what are now the seats of volcanic action. Where the vol- 
canic form is wanting, the substances lying upon the surface often 
approach so nearly in their character to lava and the other pro- 
ducts of burning mountains, as to warrant the belief that they had 
a similar origin, and were poured in a melted state from the inte- 
rior of the earth. 
4. Volcanoes are not dispersed irregularly over the surface 
of the globe, but arranged in systems or clusters, the members 
of which are commonly situated along a line either straight 
or moderately curved, suggesting the idea of their being placed 
over a rent or fissure in the crust of the earth. Nearly all the 
known active volcanoes are on islands or in the immediate 
neighborhood of the sen. Whether the same was once true of 
such as are now extinct we cannot tell, by reason of the great 
changes that have taken place, in the relative positions of the 
land and water. 
5. The following statements will serve to convey a more inti- 
mate and accurate knowledge of the nature, modes, and effects of 
volcanic action. 
Volcanoes are found to present three principal sorts of phase 
or peculiarity of character. 1. Some are remarkably uniform in 
all the phenomena they exhibit. The amount of their activity 
is the same, and it is always exerted in the same way. Stromboli 
