74 CAUSES OF GEOLOGICAL CHANGES. 
foundations. Sometimes a large part of the mountain disappears 
at once, being either blown into the air, or engulfed in an abyss 
beneath. The beds of lava that have obstructed the vent are 
broken up by the expansive force of the elastic fluid that is strug- 
gling to escape, and projected to a great height above the crater. 
The steam that rises is collected into a cloud, which overhangs 
the mountain, and condensed into rain, deluging the circumjacent 
country. It is highly charged with electricity. Vivid, violent 
flashes of lightning dart from it, and frequently occasion mischief. 
The fragments of scoriae that are thrown into the air, being shat- 
tered by their explosion, and fall, and comminuted by their 
mutual friction, are reduced at length to a fine powder, which 
mixing with the watery vapour, adds to the blackness of the 
cloud that overhangs the mountain, and either descends under the 
form of volcanic ashes upon the neighbouring country, oris car- 
ried to a distance by the winds. This substance is most abundant 
towards the close of the eruption, and frequently collects in large 
quantities in the crater so as to go far towards filling up and ob- 
literating it. In the meantime the liquid lava urged on by the 
vapour that is struggling for vent, either boils over the edge of 
the crater, or finds a passage lower down, through the side of the 
mountain. \1 1 the phenomena become gradually less remarkable 
and violent, and the eruptions after having continued for a num- 
ber of weeks or months cease altogether. It is observed that 
when the lava flows freely the tremors of the earth and the explo- 
sions become less frequent. 
40. Before proceeding to the causes of the phenomena just de- 
scribed, it will be of ad vantage to notice certain others that are near- 
ly related to them, so far as respects their origin, though they differ 
widely in their effects. There are strong reasons for believing 
that volcanoes and earthquakes proceed from a common cause, 
whoseoperationsareso modified by the diversity of the circumstan- 
ces under which they occur, as to produce the observed variety 
of result. 
Humboldt giving an account of the earthquake which shook, 
but without injuring, the city of Cumana, during his visit there, 
well describes the sensations of the man who feels the earth agi- 
tated beneath his feet for the first time. 
"From our infancy the idea of certain contrasts, fixes itself in 
"our minds; water appears to us an element that moves, earth a 
"motionless and inert mass. These ideas are the effect of daily 
" experience, they are connected with everything that is trans- 
emitted to us by the senses. When a shock is felt — when the 
"earth is shaken on her old foundations which we had deemed 
"so stable, one instant is sufficient to destroy long illusions. It 
tl is like awakening from a dream, but a painful awakening. We 
" feel that we have been deceived by the apparent calm of nature; 
"we become attentive to the least noise, we mistrust for the first 
" time asoil on which we had so long placed our foot with con- 
