streams, b, which have been ejected from the crater or from fissures, and flowed down the sides of the cone; and, 3. Masses of lava, c, filling up cracks in the cone, called "dikes." Most volcanic mountains are built up of these three kinds of material, but with varying arrangement and proportions. One kind very often predominates
almost to the exclusion of the others. The materials falling through the air upon the surface of the mountain assume a stratified arrangement, in which the finer matters are sorted out and carried to a greater distance than the others, the same as when the deposit is made from water. When materials of a different character are thrown out by different eruptions, the distinctions are very plainly marked.
An opportunity was given for observing the formation of a volcanic cone through all its stages in the case of the eruption of Monte Nuovo, or New Mountain, on the shores of the Bay of Naples, in 1538 (Fig. 7.)
After the neighborhood had been affected by earthquakes for more than two years, on the 29th of September, at eight o'clock in the morning, to be exact, water, first cold, afterward becoming tepid, was observed to issue from a depression which was noticed on the site of the future hill. Four hours afterward the ground was seen to swell