By this resistless degradation, a volcanic district is step by step stripped of every trace of its original surface. All that the eruptions did to change the face of the landscape may be entirely obliterated. Cones and craters, ashes and lavas, may be gradually effaced. And yet enough may be left to enable a geologist to make sure that volcanic action was once rife there. As the volcano marks a channel of direct communication between the interior of the earth and the atmosphere outside, there are subterranean as well as superficial manifestations of its activity, and while the latter are removed by denudation, the former are one by one brought into light. The progress of denudation is a process of dissection, whereby every detail in the structure of a volcano is successively cut down and laid bare. But for this process, our knowledge of the mechanism and history of volcanic action would be much less full and definite than happily it is. In active volcanoes the internal and subterranean structure can only be conjectured; in those of ancient date, which have been deeply eroded, this underground structure is open to the closest examination. By gathering together evidence of this nature over the surface of the globe, we learn that abundantly as still active volcanoes are distributed on that surface, they form but a small fraction of the total number of vents which have at various times been in eruption. In Italy, for example, while Vesuvius is active on the mainland, and Etna, Stromboli and Volcano display their vigour among the islands, there are scores of old volcanoes that have been silent and cold ever since the beginning of history, yet show by their cones of cinders and streams of bristling lava that they were energetic enough in their day. But the Italian volcanic region is only one of many to be found on the European Continent. If we travel eastward into Hungary, or northward into the Eifel, or into the heart of France, we encounter abundant cones and craters, many of them so fresh that, though there is no historical record of their activity, they look as if they had been in eruption only a few generations ago. But when the geologist begins to search among rocks of still older date than these comparatively recent volcanic memorials, he meets with abundant relics of far earlier eruptions. And as he arranges the chronicles of the earth's history, he discovers that each section of the long cycle of geological ages has preserved its records of former volcanoes. In a research of this kind he can best realize how much he owes to the process of denudation. The volcanic remains of former geological periods have in most cases been buried under younger deposits, and have sunk sometimes thousands of feet below the level of the sea. They have been dislocated and upheaved again during successive commotions of the terrestrial crust, and have at last been revealed by the gradual removal of the pile of material under which they had lain.