The numerous and violent eruptions from Mount Vesuvius during the two last centuries seem to afford an answer to those who think there are traces of a gradually diminishing activity in the earth’s internal forces. That such a diminution is taking place, we may admit; but that its rate of progress is perceptible—that we can point to a time within the historical epoch, nay, even within the limits of geological evidence, at which the earth’s internal forces were _certainly_ more active than they are at the present time—may, I think, be denied absolutely. When the science of geology was but young, and its professors sought to compress within a few years (at the outside) a series of events which (we now know) must have occupied many centuries, there was room, indeed, for the supposition that modern volcanic eruptions, as compared with ancient outbursts, are but as the efforts of children compared with the work of giants. And accordingly, we find a distinguished French geologist writing, even so late as 1829, that in ancient times ‘tous les phénomènes géologiques se passaient dans des dimensions _centuples_ de celles qu’ils présentent aujourd’hui.’ But now we have such certain evidence of the enormous length of the intervals within which volcanic regions assumed their present appearance—we have such satisfactory means of determining which of the events occurring within those intervals were or were not contemporary—that we are safe from the error of assuming that Nature at a single effort fashioned widely extended districts just as we now see them. And accordingly, we have the evidence of the distinguished geologist, Sir Charles Lyell, that there is no volcanic mass ‘of ancient date, distinctly referable to a single eruption, which can even _rival_ in volume the matter poured out from Skaptâr Jokul in 1783.’