At a little distance from this you come to the brink of a cliff, and looking over it, you see twelve large ponds or basins of black mud, boiling, and splashing, and raging perpetually, with large volumes of vapour rolling off their surfaces. Beyond these some dismal black-looking mountains make a back ground. Can you imagine any thing more fearful to look upon? Besides these, there is a kind of Geyser of mud, which just resembles the real Geysers in its action; but instead of sending up beautiful clear water, it throws out black unsightly mud, as thick as porridge, which is caught in a nearly round reservoir of more than one hundred feet in diameter. It is situated near the summit of a mountain called _Krabla_ and the reservoir seems to have been in time past a volcanic crater. HOW THE GEYSERS MAY BE CAUSED. Of course there cannot be hot water without some source of heat, and there must, therefore, be a source of heat taken into consideration in the action of the Geysers. Now, I do not wish to speak of this here, I only wish to make you understand how it seems likely that the eruptions of the Geysers are occasioned, supposing there was a supply of hot water. [Illustration] This cut is intended to represent what may be a section of the subterranean reservoir, S W, of the great Geyser, and the pipe, P, connected with the saucer-shaped basin, B, at the top of the mound. You see the cracks in the rock through which you must suppose hot water constantly trickling into the reservoir. The space S will be constantly full of steam, and the space below, W, will be always full of water. The water will continue to rise till it gets to A, and it will then quite stop up the escape of the steam. The steam in S will then press upon the surface of the water, and force it up the pipe P with more or less violence, according to the supply of heat; but when enough of the steam has escaped to render the pressure less, and the water has sunk to near I, the eruption will cease. If you will take the trouble to understand this section, you will see how reasonable it is that the Geyser should be so constructed; and I have seen a little apparatus made of glass, which showed exactly the same sort of operations. 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:-- [Illustration]