But a more careful study of the rocks in question will probably cause a geologist to pause before jumping to such a conclusion. If we look at the vast masses of volcanic materials erupted in Miocene times in our own island and in Ireland, for example, we might be led to imagine that we have the indications of a veritable 'Reign of Fire,' and that the evidence points to a condition of things very different indeed from that which prevails at the present day. But in arriving at such a conclusion we should be neglecting a most important consideration, the disregard of which has been the fertile parent of many geological errors. Many independent lines of evidence all point to the inference that these volcanic ejections are not the result of one violent effort, but are the product of numerous small outbreaks which have been scattered over enormous periods of time. When we examine with due care the lavas, tuffs, and other volcanic ejections which constitute such mountain-masses as those of the Hebrides, of the Auvergne, and of Hungary, we find clear proofs that the ancient Miocene volcanoes of these districts were clothed with luxuriant forests, through which wild animals roamed in the greatest abundance. The intervals between the ejections of successive lava-streams were often so great, that soils were formed on the mountain-slope, and streams cut deep ravines and valleys in them. The island of Java is situated near the very heart of what is at the present day the most active volcanic centre on the face of the globe, yet vegetable and animal life flourish luxuriantly there, and the island is one of the richest and most fertile spots upon the face of the globe. Not all the terrors of occasional volcanic outbreaks will ever drive the Neapolitan vine-dressers from the fertile slopes of Vesuvius, for its periods of repose are long, and its eruptions are of short duration. These considerations lead the geologist to conclude that the evidence afforded by the ancient volcanic rocks is clear and positive in support of the view that the manifestations of the subterranean forces in the past agree precisely in their nature and in their products with those taking place around us at the present time. On the question of great secular changes having occurred in the amount of volcanic energy in past geological periods, the evidence must be pronounced negative, or at the best doubtful. But even if the geologist confesses himself unable to establish the fact of any decline in the subterranean energies during the vast periods of which he takes cognisance, it must be remembered that such decline may really be going on; for vast as was the duration of the geological epochs, they probably constitute but a fraction of those far grander periods which are required by the speculations of the physical astronomer. CHAPTER X. THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE.