VOLCANOES. 69 
39, — 5. The cause of volcanoes and earthquakes, which, as 
they are now regarded as having a common origin, will be 
treated of under the same common title. 
Of the five causes of geological phenomena now in active opera- 
tion, this will require by far the most careful and extended con- 
sideration, for two reasons. First; because the changes produced 
by it in the crust of the earth within the period to which history- 
ascends, are by no means inconsiderable, and secondly, because 
the evidence is constantly accumulating of its having been the 
principal agent in the production of the existing continents, and 
communicating to the earth the most remarkable features by 
which its exterior surface is distinguished. 
*fl volcano is an opening made by subterraneous fire in the 
outer crust of the earth, through rohich are ejected, vapour, 
smoke, and stones, with streams of melted rock called lava. 
Some volcanoes throw out boiling water and mud. 
At the instant of its foimation, every volcano appears to have 
been simply an opening in the crust of the earth; but the stones 
that are thrown out, being urged onward by a considerable pro- 
jectile force, and their path declining more or less from a perpen- 
dicular, many of them fall at a distance from the orifice through 
which they issued, so that at first a small hillock and eventually 
a mountain, sometimes of gigantic dimensions is formed — of 
which the small ant hills that appear everywhere in a dry soil in ' 
the summer, furnish a good miniature representation. The form 
of both is the same; a truncated cone with a funnel shaped cavity, 
scoped out in its top: the inclination of both its interior and ex- 
terior surface being such as is just sufficient to prevent a fragment 
of the material of which the whole pile is built, from rolling to 
the bottom where it is placed upon the dividing ridge. 1 1 may be ap- 
propriate to mention also, that an ant hill an inch in height, and 
having the form of iEtna, would occupy about the same relative 
space in a field of seven acres, that .ZEtna does upon the surface of 
the globe. The whole mass of a volcanic mountain is not, how- 
ever, in all cases made of materials that have been projected into 
the air, and afterwards fallen to the earth. The melted lava 
sometimes rises in the cavity or crater, so as either to fill it to the 
brim and pour over its top, or after ascending to a certain height 
finds a passage through its side. In either case, after flowing to 
a distance determined by the degree of its fluidity, and the sur- 
face of the ground near the base ot the mountain, it is consolidated 
into a rock, adding something to the general elevation of the dis- 
trict in which the volcano is situated. Sometimes also it tears 
away and scatters over the adjacent country one of the sides of 
the volcanic cone, destroying altogether the symmetry and beauty 
of its form. The following are the additional facts respecting 
volcanoes, which may be regarded as more particularly worthy 
of attention. 
