other warm and mineral springs; but in connection with this subject it must be remembered that our country occasionally participates in great earthquake-vibrations, like that which destroyed Lisbon in the year 1759. [Sidenote: ANCIENT VOLCANOES IN OTHER DISTRICTS.] If we were to study any other part of the earth's surface, we should arrive at precisely the same conclusion as those to which we have been conducted by our examination of the British Islands--namely, that during past geological times the subterranean forces had made themselves felt in the area, had gradually attained a maximum, and then as gradually declined, passing through all those varied cycles which we have described in a former chapter. And we should also find that these periods of volcanic activity alternated with other periods of complete quiescence which were of longer or shorter duration. But on comparing two different districts, we should discover that what was a period of volcanic activity in the one was a period of repose in the other, and _vice versâ_. From these facts geologists have been led to the conclusion which we have already enunciated--namely, that the subterranean forces are in a state of continual flux over the surface of the globe. At one point of the earth's crust these forces gradually gather such energy as to rend asunder the superincumbent rock-masses and make themselves manifest at the surface in the series of phenomena characteristic of volcanic action. But after a longer or shorter interval of time--an interval which must probably be measured by millions of years--the volcanic forces die out in that area to make their appearance in another. Hence, although we may not be able to prove the fact by any mathematical demonstration, a strong presumption is raised in favour of the view that the subterranean energy in the earth's crust is a constant quantity, and that the only variations which take place are in the locality of its manifestation. Upon this question whether the amount of this subterranean energy within the earth's crust is at the present time increasing, stationary, or declining, we are not altogether destitute of evidence. There are some considerations connected with certain astronomical hypotheses, to which we shall hereafter have to refer, that might lead us to entertain the view that the subterranean activity was once far greater than it is at present, and that during the long periods of the earth's past history it has been slowly and gradually declining. And those who examine the vast masses of igneous materials which have been poured out from volcanic vents during the earlier periods of the earth's history may be inclined, at first sight, to point to them as affording conclusive proof of this gradual decline. [Sidenote: SUPPOSED DECLINE OF VOLCANIC ACTION.]