5. On the Borders of the Atlantic and Elsewhere.—It is an interesting fact that there are no volcanoes on the eastern borders of the Atlantic north of the West Indies Island chain. In the South Atlantic the only volcano on the borders is one of the Cameroons Mountains. In the Atlantic Ocean we have Iceland, the Azores, the Canaries, Cape Verde, Ascension, St. Helena, and Tristan d'Acunha.
This curious distribution of the volcanoes of the world near the oceanic waters appears to be dependent rather on the very early shapes of the continents and the ocean beds than on their present shapes.
[CHAPTER XVII]
VOLCANOES OF THE GEOLOGICAL PAST
The question is often asked whether the volcanic eruptions of the geological past were not much more violent and destructive than the volcanoes of the present time. Now, while this is a matter that properly belongs to the subject of geology, and will be treated at greater length in the Wonder Book on Geology, yet a short mention should be made of it here.
It is the opinion of Dana that while there have been volcanoes during the different geological ages, yet volcanic activity has increased through the geological past until the age that immediately preceded the appearance of man on the earth. He thinks there is no reason for believing that there were any very great volcanic eruptions during the earliest geological time known as the Archæic. Dana speaks as follows concerning this:
"In this connection it is an instructive fact that in eastern North America, at epochs when there was the greatest amount of friction and crushing ... those of the making of the Green Mountains and the Appalachians ... no volcanoes were made, and little took place in the way of eruptions through fissures."
On the other hand, Prestwich seems inclined to think that the absence of well-marked cones of volcanic material in the rock of the older geological ages is not to be regarded as proof that no eruptions then took place, since the very great amount of erosion that occurred between that time and the Tertiary Age before the appearance of man, would, probably, have completely obliterated any cones, and even the volcanic materials would have undergone such changes as completely to alter their general character. He agrees, however, with Dana that, probably, the most violent and explosive volcanoes of the geological ages have been those of the Tertiary Age.
Without, however, attempting anything more than a brief reference to the volcanoes of the geological past, it may be said that many of the more important of the active volcanoes of the earth's present time were begun in the Tertiary Age. Mt. Etna, Vesuvius, and Mt. Hecla are believed to have commenced at this time.