314 Professor Jameson mi the Rocks formed hy HoUSprings, 
This air-volcano is usually in the state wejbave just described 
it, but not always, for it is occasionally very much agitated, and 
exhibits phenomena of a more striking and extensive nature. 
When this is the case, loud internal noises are heard, and these 
are accompanied with sharp shocks of earthquakes, which are 
felt to a distance of two or three miles, and these noises and 
earthquakes continue more or less for several days, and do not 
cease until an eruption takes place of an enormous jet of clay, 
earth, and stones, which are thrown into the air the height of 
two or three hundred feet. The explosions are repeated two or 
three times in the space of twenty-four hours. There are years 
when no eruption takes place. The following is an account of 
one of these eruptions as given by an eye-witness. A dull noise 
was heard ; the earth in the neighbourhood was much agitated, 
and large rents opened. There arose from the centre of the 
plain, a bubble of diluted clay, sixty feet in diameter, which gra- 
dually increased in magnitude, and attained the enormous ele- 
vation of two hundred and thirty feet. This extraordinary 
and striking spectacle continued for half an hour ; was repeated 
three times, with an interval between each of a quarter of an 
hour ; and, during its continuance, a stormy noise, like that 
of the sea agitated by a tempest, was heard to proceed from 
below. The author of this account seized the first moment to 
approach the point from whence the explosion proceeded ; he 
plunged his arm into a place where the ebullition was still vi- 
sible, but did not feel any increase of temperature ; on the con- 
trary, the mud felt cold. 
There are many small mud-volcanoes in the neighbourhood of 
Modena, whose height is not more than a few feet. They are 
named Salses, on account of the saltness of the water they throw 
out ; and which, indeed, is also the case with the water of Maca- 
louba, and of that of most other muddy eruptions observed in dif- 
ferent countries. These volcanoes, during their paroxysms, 
which are attended with shght agitations of the earth, throw out 
much mud, which extends to the distance of three thousand 
feet. The gas which occasions the eruptions, is sulphuretted 
hydrogen mixed with petroleum, and a little carbonic acid. 
Similar air-volcanoes are described by Pallas as occurring 
in the Crimea, particularly in the Island of Taman. In the 
year 1794, one of these burst with a noise like that of thunder. 
