Some little variation in the shape of the reservoir and pipe, may perhaps be needed to account for all that takes place; but the principle of the activity being produced by the agency of steam, acting in a space which may be enclosed by the water in its rising to a certain height, seems to be certain and satisfactory.
CHAPTER VII.
OF THE ROCKS CALLED BASALTIC.
In some parts of the world, there are rocks which are not stratified and arranged like those which I described to you some time ago, but are laid over the surfaces, and in the crevices of others in this manner:—
In this cut, a, a, a, are intended to represent the stratified rocks, and b, b, b, the others, which from their position are sometimes called overlying rocks, and they mostly consist of a substance called basalt, which is nearly black and very hard, and occasionally with white and coloured minerals imbedded in it. There are two states in which it is found; in one, it occurs in masses without any particular form, and in the other state it constitutes pillars shaped with great accuracy and regularity.
